Cockpit drains draining below the water line are a funny
thing. My current
boat has the same set up as yours and I discovered not long after buying
it that the single
hose clamp clamping the hose to the thruhull had missed the nipple altogether. Goodness knows how long it had been like that for, but the hose had never popped off, so in a way that kind of instills some confidence that the drains will remain intact.
Now back to your question, water would enter the cockpit regardless of outlet height. It is a function of cockpit floor height above the waterline. This also means heeled waterline as well. If the "fall" between the drain and outlet is not enough though, you may have problems with draining in a seaway as the water will slosh back into the cockpit before it escapes the drain plumbing
. Seas can also enter the drain if they rise higher than the floor with the boat's motion, but I guess rubber flap valves could be used to prevent this.
Your sink drain through hull
only needs to suit the size of the drain. My washbasin drain in 3/4" which drains a bit slow, but at least you cant loose your wedding ring down it. Perhaps the larger through hull
has been repurposed from a previous function e.g. toilet outlet.
No one like lots of underwater through hulls, but plenty of boats have them. If you maintain them I really can't see why they would need to be repositioned. I personally wouldn't be bothered to move mine.