Re: Is it common for a livewell to to drain through the boat's rear drain plug?
I know some boats have a "cooler" compartment for drinks or food that drains to the bilge. Not the best design, but definitely not supposed to be used as a livewell.
That design difference is based on whether the bottom of the box is above or below the waterline. Or it should be; the real reason in cheap boats is cheapness.
So I have a "bay boat" hull with a floor drain into the bilge, to be pumped out; live well with intake and drain below the water line which makes me nervous, and small drains in the two lockers that drain into the bilge. The latter two I suspect drip onto flotation foam but if designed correctly they would be hosed at least to the keel.
On my center console the anchor locker, two fish boxes and live well each has its own drain overboard above the line, plus 2 scuppers aft to drain the cockpit. The only drain into the bilge is in the bottom of the console, which is not designed to get wet, but for just in case.
The cup holders have drains even that have tubes and fittings that go to the deck. Rod holders drain onto the deck.
Point is, thsi is how the rot-inducing sweetwater can get below--but it's not a problem as long as you keep the boat drained. On the trailer, bow up and plug out. Moored, bow up and float switch on the bilge pump; the pint that remains in the very stern won't hurt anything.