From a quick search:

• !kubesail@lemmy.world
• !royal_caribbean@lemmy.world This is a place to learn about sailing on Royal Caribbean ships and ask questions.
• !sail@lemmy.world
• !sailing@lemmy.ml
• !sailing@lemmy.world General\n\nWelcome! This is a place to discuss sailing
• !sailing@reddthat.com Let’s talk about boats
• !boat@lemmy.world
• !boating@reddthat.com A community for boating enthusiasts
• !boats@lemmy.world Lemmy community for boats.
• !jboat@lemmy.sdf.org Talk about owning
• !narrowboat@feddit.uk A community for UK inland waterways enthusiasts. While boating is our main topic. Any UK canal or river based discussion is allowed.\n\nRules
• !precious_metals@lemmy.world A support group for survivors of boat accidents.\n\nRules

Good luck! :)