Last year I bought a gallon of Home Defense from Sams club which comes with a sprayer etc. I sprayed under all railings, corners, vents, ropes/cleats, hatches and even corners and an outline of the floor inside the houseboat. It did not stain or mess up any of my fiberglass, canvas etc. It worked great for longer than I expected. I did notice some come back later in the summer but not even close to as many as I had before spraying in the spring. I only used half of it last year so just used the rest last week for this year. We'll see if it works that well again! Good luck!
I'd keep a spray can of Pyrethrins on board. I was an exterminator for years. I had a lot of boats I took care of on a regular basis. Flotation foam is great for ants. Tractor supply has a can about 20 ounces called stable fly spray or something like that. Look for the active ingredient. You have to be careful around water.
I child proofed the house but still keep getting in.