Re: Self Bailing
on a 17' boat you don't need 2 batteries, especially if you keep it on a trailer.
With floor drains you really have to have a float switch. it will not run down the battery.
They are set (or should be set up) to let a couple inches accumulate, then they kick on, and stay on until the boat is about dry. They will also kick on when you accelerate to get the rest of the water out, so mount as close to the transom as you can.
I would not drill holes in the back for self-bailing; I think if it were feasible the boat would be made that way. You probably don't have enough clearance and in fact your deck may be below the waterline to begin with. Level scuppers don't drain a sitting boat very quickly and could be overwhelmed by a big rain or waves over the stern.
I have a 1988 17' Key West DC with the floor drain, too. 1 battery, float switch, left the boat in the water for years no problem, but it was at the house so we could keep an eye on it. Over the years, we had a pump fail and a battery go down (short) and even though we had heavy rain and water above the drain, we pumped it out no problem.
If you leave the boat in the water, get a battery switch so you can cut off all power except the float switch, which will be wired direct.
Suggest you buy a hand pump; you can take the cover off the floor drain and stick it in the bilge for emergencies/equipment failure.