We’re so glad you asked. For boats with dual batteries, here’s one we recommend. This will charge, and maintain two separate batteries.
https://www.amazon.com/NOCO-GEN5X2-Fully-Automatic-Temperature-Compensation/dp/B08G3STB1N/
If you have a single battery, you just need the single bank,
https://www.amazon.com/NOCO-GEN5X1-Fully-Automatic-Temperature-Compensation/dp/B08G3NVR1N/
Either way, get it. It’s the best investment you’ll ever make in your boat, to keep it running well!