Hey,
I don't know if anyone has tried this before. But, rather than using a mirror, or trying to get your whole head inside a 5" porthole, why not use a digital camera.
I just did it to check out a new captive bolt assembly that I put into a nacra for someone and it worked great. Just put a piece of masking tape over most of the flash or it will be too bright.
Pretty cool, you can get a perfect shot of some inside detail with perfect lighting.
I did make some Pictures using a Canon IXUS digital camera.
It works pretty good. see Link
I can send some more pictures from either a NACRA 6.0 or a A-CAT ( pesa )
GOOD IDEA!
WHAT WOULD I BE INSPECTING THE INSIDE OF THE HULLS FOR?? I HAVE A HOBIE 18 FROM 1985 WHICH IS IN GREAT SHAPE. THE OTHER DAY I WAS LOOKING INSIDE THE HULLS (DURING THE DAY) AND NOTICED THAT THE WHOLE BOTTOM SIDE OF THE HULLS WERE SOMEWHAT TRANSLUSCENT. THE LIGHT COMING THROUGH WAS NOT SPOTTY OR PATCHY, IT WAS EVEN FROM FRONT TO BACK AND CUT OFF ABOUT A COUPLE OF INCHES UP. THERE THE SIDES OF THE HULLS BECAME TOTALLY OPAQUE. I LOOKED AT THE HULLS FROM THE OUTSIDE AND ASIDE FROM SOME MINOR SCRATCHES THE HULLS WERE FINE. ANYONE KNOW IF THIS IS NORMAL OR WHY THIS IS??
that's normal. The sides are foam cored so they block light a lot better. The foam does not continue down to the bottom and since there is no seam in the hull bottom (like Nacras) you're just looking through several layers of fiberglass there.
THANK YOU JAKE, I GOT A LITTLE WORRIED THERE FOR A SECOND THINKING THAT THE BOTTOM OF MY HULLS WERE ONION SKIN THIN. 
on my boat, with half inch foam core, you could read the name from the inside of the boat on a sunny day.
glass is translucent, regardless of thickness. in most cases if the gel is not there, you will see light through. i could see light
through the bottom seam on a pdq 36. the boat is about 5/8 inch thick there.
while working on spirit of canada, an open 40 from the around alone, we were leaving the shop for the day. with the lights
off in the shop, we could clearly see some one had left on a light in the boat. again through the core.
Yes those cameras are handy tools, aren't they! Just the other day I was staning on the beach and someone asked me about something inside the hull and I grabbed a nearby friend who had a camera in his hand and within seconds, we were looking at an image of the inside of the hull.
I always thought that it would be cool to get one of those cameras that the plumbers use for drains and be able to send it up the hull, past the bulkheads!
Last evening I snapped the attached photo of the upper gudgeon plate on my Mystere 6.0. The green line is on the deck plate. You can see the metal reinforcement plate around the two machine screws. It looks like we can see the gudgeon pin right through the transom.
GARY
Sure beats the way we used to have to do it on the Shark. When we had to inspect or repair something inside the hull, my sister had to go in -- and I mean literally into the hull. That was a drawback to being the smallest one in the family.