WaterSplat-SLAM: Photorealistic Monocular SLAM in Underwater Environment

IEEE Robotics and Automation Letters (RA-L), 2026

Kangxu Wang^* · Shaofeng Zou^* · Chenxing Jiang^* · Yixiang Dai · Siang Chen · Shaojie Shen · Guijin Wang^†

Underwater monocular SLAM is a highly challenging problem with applications ranging from autonomous underwater vehicles to marine archaeology. However, existing underwater SLAM methods struggle to generate high-fidelity rendered maps. We propose WaterSplat-SLAM, the first novel monocular underwater SLAM system to achieve robust pose estimation and photorealistic dense map construction to our knowledge. Specifically, we combine semantic medium filtering with a dual-view 3D reconstruction prior to achieve underwater adaptive camera tracking and depth estimation. Furthermore, we propose a semantically guided rendering and adaptive map management strategy, combined with an online medium-aware Gaussian map, to model the underwater environment in a photorealistic and compact manner. Experiments on multiple underwater datasets demonstrate that WaterSplat-SLAM achieves robust camera tracking and high-fidelity rendering in underwater environments.

Video Demos

The following videos show online SLAM reconstruction and rendering results. The Pool Loop sequence highlights the loop-closure behavior of WaterSplat-SLAM.

Panama Sequence

HI-SLAM	MonoGS	WaterSplat-SLAM (Ours)

Pool Loop Sequence

HI-SLAM	WaterSplat-SLAM (Ours)

Installation

Prerequisites

• Linux (tested on Ubuntu 20.04)
• Python 3.11
• CUDA 11.8+ (tested with CUDA 11.8)
• GPU with at least 24GB VRAM (16GB+ recommended)

Step 1: Clone the Repository

git clone https://github.com/THU-VCLab/WaterSplat-SLAM.git --recursive
cd WaterSplat-SLAM

If you already cloned without --recursive, initialize the submodules:

git submodule update --init --recursive

Step 2: Create Conda Environment

conda create --name watersplat-slam python=3.11
conda activate watersplat-slam

Step 3: Install PyTorch

Install PyTorch with the appropriate CUDA version:

# CUDA 11.8
pip install torch==2.5.1 torchvision==0.20.1 torchaudio==2.5.1 --index-url https://download.pytorch.org/whl/cu118

# CUDA 12.1
pip install torch==2.5.1 torchvision==0.20.1 torchaudio==2.5.1 --index-url https://download.pytorch.org/whl/cu121

# CUDA 12.4
pip install torch==2.5.1 torchvision==0.20.1 torchaudio==2.5.1 --index-url https://download.pytorch.org/whl/cu124

Step 4: Install Third-party Dependencies

pip install -r requirements.txt

# Install MASt3R (3D reconstruction backbone)
pip install --no-build-isolation -e thirdparty/mast3r

# Install in3d (visualization utilities)
cd thirdparty
pip install --no-build-isolation -e in3d
cd ..

# Install fused-ssim (CUDA-accelerated SSIM loss)
pip install --no-build-isolation thirdparty/fused-ssim

# Install simple-knn
pip install --no-build-isolation thirdparty/simple-knn/

# Install lietorch (Lie group operations)
pip install --no-build-isolation lietorch@git+https://github.com/princeton-vl/lietorch.git

Step 5: Build SLAM Backend (CUDA extensions)

pip install --no-build-isolation -e .

This compiles the Gauss-Newton solver CUDA kernels for the SLAM backend.

Step 6: Build Water Gaussian Renderer

cd water_gaussian
pip install --no-build-isolation -e .
cd ..

This compiles the cudalight CUDA module for underwater-adapted Gaussian rendering.

Step 7: Install tiny-cuda-nn

Required for the medium MLP network encoding:

pip install ninja
git clone --recursive https://github.com/NVlabs/tiny-cuda-nn
cd tiny-cuda-nn
cmake . -B build -DCMAKE_BUILD_TYPE=RelWithDebInfo
cmake --build build --config RelWithDebInfo -j
cd bindings/torch
python setup.py install
cd ../../..

Step 8: Download Checkpoints

Download the MASt3R pretrained weights and retrieval codebook:

mkdir -p checkpoints/
# MASt3R model weights
wget https://download.europe.naverlabs.com/ComputerVision/MASt3R/MASt3R_ViTLarge_BaseDecoder_512_catmlpdpt_metric.pth -P checkpoints/
# Retrieval model weights
wget https://download.europe.naverlabs.com/ComputerVision/MASt3R/MASt3R_ViTLarge_BaseDecoder_512_catmlpdpt_metric_retrieval_trainingfree.pth -P checkpoints/
# Retrieval codebook
wget https://download.europe.naverlabs.com/ComputerVision/MASt3R/MASt3R_ViTLarge_BaseDecoder_512_catmlpdpt_metric_retrieval_codebook.pkl -P checkpoints/

Download the CLIPSeg weight for water segmentation:

wget https://owncloud.gwdg.de/index.php/s/ioHbRzFx6th32hn/download -O weights.zip
unzip -d weights -j weights.zip
rm -rf weights.zip

Datasets

We evaluate WaterSplat-SLAM on two groups of underwater datasets. After downloading and extracting them, organize the repository as shown below. The dataset directories may also be symbolic links to data stored elsewhere.

SeaThru-NeRF

SeaThru-NeRF is the CVPR 2023 dataset used for the four real underwater scenes below. Download it from the official dataset link on the project page, then arrange the extracted scenes as follows:

SeaThruNeRF_datasets/
├── 4_Curasao/
│   ├── images_wb/
│   └── sparse/0/
├── 5_IUI3-RedSea/
│   ├── Images_wb/
│   └── sparse/0/
├── 6_JapaneseGradens-RedSea/
│   ├── images_wb/
│   └── sparse/0/
└── 7_Panama/
    ├── images_wb/
    └── sparse/0/

The directory names above match the provided scene configurations. Note that JapaneseGradens is retained in the local directory name for compatibility.

WaterSplat-SLAM Dataset

Our captured dataset can be downloaded from Google Drive. Extract the archive and place the four scene directories under WaterSplat_datasets/:

WaterSplat_datasets/
├── big_gate/
│   ├── images/
│   └── sparse/0/
├── pipe_local/
│   ├── images/
│   └── sparse/0/
├── pool_up2/
│   ├── images/
│   └── sparse/0/
└── pool_loop/
    ├── images/
    └── sparse/0/

Each sparse/0/ directory must contain the COLMAP binary model:

sparse/0/
├── cameras.bin
├── images.bin
└── points3D.bin

For our captured scenes, both the camera intrinsics and the ground-truth camera trajectory were estimated using COLMAP. WaterSplat-SLAM reads the intrinsics from cameras.bin and the reference poses from images.bin.

Running

Run a scene with its corresponding configuration:

# SeaThru-NeRF
python main.py --dataset SeaThruNeRF_datasets/4_Curasao --config config/Curasao.yaml
python main.py --dataset SeaThruNeRF_datasets/5_IUI3-RedSea --config config/RedSea.yaml
python main.py --dataset SeaThruNeRF_datasets/6_JapaneseGradens-RedSea --config config/Jap_RedSea.yaml
python main.py --dataset SeaThruNeRF_datasets/7_Panama --config config/Panama.yaml

# WaterSplat-SLAM Dataset
python main.py --dataset WaterSplat_datasets/big_gate --config config/big_gate.yaml
python main.py --dataset WaterSplat_datasets/pipe_local --config config/pipe_local.yaml
python main.py --dataset WaterSplat_datasets/pool_up2 --config config/pool_up2.yaml
python main.py --dataset WaterSplat_datasets/pool_loop --config config/pool_loop.yaml

To run every available scene in both dataset roots:

bash scripts/run_our_data.sh SeaThruNeRF_datasets WaterSplat_datasets

Missing scenes are skipped by the batch script. Results are written to the output.base_dir specified by each scene configuration.

Acknowledgements

This project builds upon several excellent works:

• MASt3R-SLAM - Dense stereo SLAM backbone
• MASt3R - 3D reconstruction from image pairs
• 3D Gaussian Splatting - Neural radiance field rendering
• Water-Splatting - Underwater Gaussian splatting with medium rendering
• Lowlight Splatting Underwater - Underwater cudalight renderer
• DROID-SLAM - CUDA backend kernels
• gsplat - Gaussian splatting library

License

This project is licensed under the CC BY-NC-SA 4.0 License. See LICENSE.md for details.

Citation

If you find this work useful, please cite:

@ARTICLE{11417448,
  author={Wang, Kangxu and Zou, Shaofeng and Jiang, Chenxing and Dai, Yixiang and Chen, Siang and Shen, Shaojie and Wang, Guijin},
  journal={IEEE Robotics and Automation Letters}, 
  title={WaterSplat-SLAM: Photorealistic Monocular SLAM in Underwater Environment}, 
  year={2026},
  volume={11},
  number={5},
  pages={5614-5621},
  doi={10.1109/LRA.2026.3668465}
}