You can find it...and you should.
Just follow the 3/4" line off the back of the toilet till you get to the thru-hull. You may lose it as it goes through a bulkhead, but it'll be the only line that looks exactly like it wherever it shows up again.You'd be wise to find and follow all your plumbing and wiring...an emergency isn't the time to START looking for things like seacocks and breakers.If the toilet is original, finding and opening the seacock may not solve the problem. If it doesn't, get back to me with the make/model/age of the toilet and we'll see what it does need.