Just an hour ago my wife found videos about Koh Kra. And they told it is very nice (but it was the agency which organize the tours in a very Thai style (lots lots of "entertainment")).The diving at Koh Kra isn't all that great.
There is some very nice snorkeling with a bunch of blacktip reef sharks in 2 meter of water or less at Hin Sung.
You will not see them while on scuba (and not even when you bring a bucket with fish-guts under water with you) but they will be all around you when you snorkel.
A question is if it is reachable per longtail. It is approx. 50 km away. If a longtail makes 25 km/hour than it will work. If it makes 15 km/hour than it will not....but might be still faster than going to krabi.....
In 3-4 months I'll get my compressor and tanks than I'll check it out...