How much to tip boat captain for bachelorette cruise?

Hi all, hopefully it’s ok to post this here…. I’m in charge of planning the boat ride for a bachelorette party in Fort Lauderdale and was wondering what a normal tip for the boat captain would be? I would have to pay the captain separately, day of, and wasn’t sure if I should tip 20% of his fee or the cost of the boat rental? Or just tip an even $100? It’s for a 4-hour ride.

I have one company that I’m eyeing for this but feel free to drop any recommendations of a good party boat company in the area! Thanks so much for any suggestions!