You don't say where you are from or where you wish to ultimately go, nor do you say what kind of boat you have. So I will have to argue from general principles here, principles based on having owned two boats since 1999 and one bull terrier from 1986-2001 (R.I.P.).
Do NOT try to make sailors out of bull terriers, particularly two of them. Reasons to avoid dogs in general include the need of dogs to get daily exercise...exceedingly difficult on a pitching deck. Bull terriers do tend to sleep and fart a great deal, but they can also "fetch the tree" or "retrieve the tire" or "drag the sled" for as long as your arms can hold out. They are immensely strong for their size, and if they are not well-exercised (and not just by demolishing supposedly indestructable "Kongs"), they will likely grow peevish, fat and ill-tempered. Also, a boat is BORING for a dog...no interesting bits of dirt, trees or other dogs' puckerpoints to sniff, and insufficient cabin sole space for the classic bull terrier "spins" that regularly dislodge terrestrial furniture. And let's not forget the epic craps a pair of bull terriers can produce, along with the gas-passing and chow breath and incessant rolling to get comfortable, which will surely wear holes in your expensively Sunbrella-clad V-berth.
And let's not forget that bull terriers are solid, almost cinder-block-like animals. That egg-shaped head will take a bad tumble down a steep companionway, and if you can carry one in one arm while safely going up and down yourself, then you are a circus giant with an entertainer's pension and don't require my advice. The same brute solidity makes them poor swimmers (sinkers, really) and they suffer from inertial forces found in abundance aboard sea-going vessels. Not to mention their relatively thin coats mean they'll get easily chilled in a damp breeze...when they sneeze, I doubt you'll be carrying enough Spray Nine to remedy the carnage.
Lastly, a large number of places will quarantine dogs of any breed, and a subset of those will refuse you entry. I strongly dissuade you therefore to take your dogs, lovable as I well suspect them to be, cruising. If you do, restrict your trips to North American daysails, where every 50 miles sees a dog-friendly, restriction-free harbour where your little darlings can eat a 2 by 4 or six and have a nice, relaxing BM.
Hope this helps.