My boat is not well set up to dive from so I don't take people out on it to dive. However I do take people out to kayak and to just hang out.
If I invite you out on the boat, then I have no expectation that you will contribute - as I would have no expectation if I invite you over to my house. Bring food and booze and we are good.
However - the cost of fuel invariably comes up and as I get about 1 mile per gallon (if I go really slowly) it is an easy conversion to dollars for most to make. i.e. in Canada it costs about $5 in fuel for every mile we travel.
I gratefully accept all contributions, but in the scale of running a boat for a year a couple of hundred bucks isn't going to make a difference one way or another.
Just to give those out there who don't own a boat some idea - my parking bill for the year for a 37 foot boat is $7,000 (it is an old wooden boat so it is boathouse kept which doubles the parking bill and halves the maintenance bill and the time I have to spend on sanding and varnishing), haulout and bottom paint $1,000 per year, insurance $2,200. Maintenance ???? depends on what breaks but figure 10% of the purchase price every year and you might get change, you might not. Depends on how much you do yourself and how much you have to hire someone to do. Nothing on a boat costs less than $1,000 to repair.
As someone said if you have to worry about the cost of fuel you can't afford the boat.