I keep my boat in a slip at the local marina at Lake Waco. There are snakes in the water now and there will continue to be snakes, but I'd like to keep them away from where I am as much as possible. I've seen 2 in the water in the last week. It's a floating marina with dense foam at the bottom, an open gap (can access the top of the foam easily, then steel + concrete.

I can't keep snakes away 100% of the time but what can I do to make them less likely to come near my slip? Do you think I could spread Snake-B-Gone or sulfur or something on top of the foam layer? Would they be able to smell/sense it while they're in the water and make them go someplace else?

Diamond back water snakes. About the worst they can do is lick you with their long tongues. Don't worry about them.

Don’t go to Navarro. Mills and look under floating dock. Seen piled on top of foam by the dozen

It is not a good idea to put out chemicals near the water. Eventually the chemicals will find their way into the lake. The snakes are probably non-venomous, so they should not create a problem. The only venomous water snake in Texas is the western cottonmouth, often called water moccasin. If the snake is positively identified as a western cottonmouth, I would remove it.

1st thing that comes to my mind is chlorine tablets like the ones you put in the pool.
But I don't know if that's the best thing for so close to the water.

I hope you are just joking

Get a pet mongoose and take him to the lake with you.

Snakes, Dirt Daubers, Spiders, and Duck's that poop on everything are just a fact of life at marina's. I be careful about what you put out to keep them away.