Ann and I have cruised seasonally on the east coast
for the last 18 years or so, after crossing from Mexico
in 1990. We've done heaps of sailing in Moreton Bay and the Broadwater (well, mostly motoring there!). We've been from Lizard Island to Southwest Cape as well as annual trips to the Islands (VAnuatu, N. CAl
, Sollies. etc). And, despite the "common knowledge" that shoal draft is required for many of these cruising grounds, both of our boats have drawn 2.2 metres (7'2"). Have we run aground??? Sure we have... just slightly farther from shore than our shoal draft friends have been stuck!!!
There have been a very few places we simply couldn't get into, but I reckon that that is a small price
to pay for the added stability and windward ability that we have gained. The only sort of thing that has bothered us is the inability to get into certain mangrove creeks with bars across their mouths, thes places often being good cyclone holes. But, for many of them one would have to be really shalllow, like a metre or so. One point for the multi hull
So, my answer to your original query is 6'8" will not be a realistic problem in your proposed cruising grounds, or most other places in the world.