Re: oil change
Yes, you can use the capacity to be comfortable, but . . . This is very IMPORTANT! Only use the capacity if you know you got everything out. If you think you have it empty, but there is actually 2 quarts left, and you put in 5 quarts that could be a problem. Does it have a stick? If not you need something to use to determine if you are using any oil during regular operation . . .
As far as being in the water is concerned, if you know how your boat sits then simply approximate that. You can use a level across the waterline if you can see it and you know that it is true in the water. Close is fine, overfilling is worse than underfilling as long as there is some oil in there.