I have been diving a long time, and for the first 20 years or so, diving overlapped with sailing. My Dad had a 30 footer. Trust me in this, they don't go together. It sounds like you haven't done much sailing, so let me itemize a few problems with sailboats for diving. (And don't get me wrong, I love sailing, just not for diving.)
Sailboats are: slow, rarely able to go directly to where you want to go, covered in wires and ropes and cleats and all sorts of trip hazards, lack any sort of open space unless you are 40' plus, difficult to trailer (even a small one) so you're range is limited, and have high freeboard (with fences!) Sailboats suck for diving.
From a purely financial perspective, the money you lose selling your stink-pot and buying a sailboat, would cover your fuel for a decade. Also, if the boat is big enough to dive from, you won't be able to pull it around on a trailer easily (keel boats are VERY heavy, tall, and require dismasting to trail) so you'll have to leave it at a marina which will cost a fortune. I assume you put your little bow-rider on a trailer and away you go..
So my (good) advice is to upgrade to a four-stroke if you haven't already, drive a bit slower if you must, and stick with what you have.
I use my boat a lot (20' Zodiac Hurricane RIB) and it burns about a litre a mile (call it a US quart for you 'Mericans) which sucks, but the reality is, I don't really go very far most of the time. Most of the time, I am heading out 3 or 4 miles and back... say 8 litres or 2 US gallons. Big deal. In a typical dive season (April to December) I average about $350 - $500 in gas, which in Canada, is a lot more than you would pay. And I dive almost every weekend. (My boat lives on the trailer, so I fill up at gas stations as a rule, not marinas). If you get a good sized sailboat, your booze tab with be way more than that. Believe me, sailors are a thirsty lot and they love to sit around on their boats and TALK about sailing with (your) booze in hand, even more than sailing itself. Sailors make divers look like Baptists!
So sail if you like sailing, but leave the dive gear at home... or take up free-diving.
You're welcome.