There's not one answer to this question. It depends on so many things, there's probably a couple dozen answers.
Wookie gave you one very true side of what is having a boat for diving, but there's many other sides.
My side is very different, purchased a boat for my husband on our anniversary over a decade ago. The boat of his dreams for over 20 years. Got it brand new and within a month between both of us set things up to be a dive boat, we do some cruising, picnics and other non-diving activities but is all secondary to diving.
Besides one battery change (one set) and services every so many hours which my husband does every other one, there's been zero issues. The boat is about to be 12 years old and I can't imagine my life without it.
We use it a minimum of once a week and no less than 2 o3 times a month we go at night. The more you use it the faster you catch issues before they become problems.
Last year we did a once over where found screws starting to rust and window hinges looking crappy, speaker covers also half rusted and just too many little places making it look less than great. Took us a good week or two of mayor scrubbing, soaking, replacing and polishing. Of course that included a few trips to Sailorman and WestMarine but there were no mayor purchases, granted a 2" stainless steel screw is about $3.50 or so, but it was no where close to thousands, was mainly time and effort but also satisfaction. Husband and I enjoyed the fact that it was getting done our way.
Now because of our location we can make 7 to 8 dive trips every 100 gallons of fuel, usually get an average of 5 to 6 because we go to different places some farther than others, but if money was to get tight we'd go to the closest locations and keep diving.
We don't deal with marinas or boat ramps, have the boat secured in our backyard on a lift.
Being in south Florida we get nailed with insurance, about 800 a year but is by choice, the vessel is paid and could reduce the policy to just liability for a small fraction of that.
12 years ago we paid a bunch of money and for the last 8 years the out of pocket hasn't been that much.
There are many ways to own a boat, not all will will make you wish for the day you put it up for sale.