I would like to de-attach and re-attach the T-top of my boat. But
before I do this, I want to make sure that I will be able to re-attach
it. The problem is that I have very little access to the under-deck
area right under the legs of the T-top. There is only around 2" to 3"
space underneat the deck -- I can stick my hand into it, but I cannot
turn my hand to do any work. Therefore, I am looking for a way to
attach the T-top onto my deck from above the deck.

Ratcheting box end wrenches?

But one example:
http://www.armstrongtools.com/newproducts/geared.jsp

Either that or use an air ratchet with a shallow socket. There's probably
enough room to get one in there, possibly with the nut already in the
socket. Just keep a suitable magnet handy to fish the darn things out of
there if/WHEN they wiggle loose.