brian;
feel free anytime to call me. However as a dealer, I told my customers never to seal from inside. If something leakes, then you will want to know about it sooner than later. I knew a few who did what your are suggesting and regretted it. One reason for a coating for example gel coat is to prevent a chemical reaction over time with water and bare fiberglass. That is what happenend which repairs were done due to the sealing inside.
If you are concerned, then run a hose up to the boat and wet the area for 5 minutes or more and if nothing, you can be fine but recheck after sailing a few times. In fact, an old test to find many leaks is to close the boat up with tape over the edges of the opeing ports and forward hatch and then cover the entire companionway with plastic taped of course. Then insert a leaf blower into it taping it as well. With a person and bucket of real soapy water, turn on the leaf blower and brush everything. Where you see air bubbles escaping, there you will find leaks. Simply you are pressurizing the cabin and where air escapes you will find potential leaky areas.
It has been a long time but generally the area of the deck is generally solid glass but possibly divinacell in it. divinicell is basically foam core that shrinks when in contact with resin but is a structural strong mat that allows less weight and flexibility for example the hull.
I hope this helps but if you have any further questions, get hold of me.
