The owner of the ski also had two 1/4 size holes drilled under each side of the tray left unplugged. That surely accelerated the sinking time.
After surviving several low/high tides, the ski traveled probably 400-500 yards farther out from where it went down on the bottom of the ocean with a full fuel 5gal tank as the only positive buoyancy (besides the fiberglass) in it. The fuel had completely ran out by the time we found the ski. All local experts had completely false prediction that the ski would get beached to the shore over night.
That event raised the following conclusions:
Ocean is absolutely unpredictable. A sunk ski can end up two miles away or stay exactly where it has sunk. No one can predict that.
Best way to spot a sunk ski in the ocean is by helicopter in clear (high tide) waters (the first time it was found) or sonar and 10+ people searching for it (the second time it was found).
It might sound absurdly, but chances to find a sunk ski are better if the ski has no floatation at all and goes straight down to the bottom and stays there, or it has enough floatation to float or bob around until it is rescued. If its in between, the ski will be gone with the current within hours.
A month ago a square nose sank in the same place, losing its hood but it bobbed around until it got beached within 30-60 minutes. That makes me think that a fully foamed tray even water logged is safer than no foam at all.
Scupper is rather not a good idea for a surf skis. I am planning to plug mine off.
Reliable bilge pump is a must for a surf ski.
Bow eyes on back and front of a surf boat are must.
Every surf rider in the group must carry a tow rope, not just one out of 10. It also has to be easily accessible or its useless.
If ski gets filled with water and goes under, there is no chance to be towed by another standup. I have tried that twice and almost ended up sinking myself too. Either let it bob around until better rescue shows up or wrap a life jacket around the pole and pray for a good outcome.