Good point about the lines exiting above. However, there are several cables that continue to the step (anemometer, nav lights, deck lights, radar, VHF antenna, TV antenna). I don't think plugging it up would be a good idea.
As i said -there are solution for that also..
each cable get its own piece of electrical tubing matching the cable diameter (as close as you can - considering plugs on the ends
let these pipes extend 20cm (8 inches) above the internal plug in the mast.
Only the water running along the cables will enter the bilge, the rest drains out of the mast.
My mast had an internal plug like i described.
The PO made a hole in that plug to get the VHF cable down w/o sealing the hole.
When we had the mast down we
a) inserted a piece of electrical tubing for the VHF cable
b) Made a new hole with a tube for the new TV antenna cable
Not very difficult but need the mast down
The sealing compound has to be poured with the mast in place.