KJ,
The hatteras 53 is an endearing classic that has been always well regarded and accepted in the boating world as a very good boat, adequate power, and great layout. I would not say they are under powered at all but it depends on what you want to do with it.
if you are doing a lot of diving, though I am not a diver yet myself, a boat with a cockpit is just about essential. That could be a cockpit motor yacht like the Carver RT posted or another model. The 47 POC defever is a great boat (been on a few of them)- just probably not at all what you would want for a diving boat. Most of the 47 POC's have no davit for dinghy storage, they are an aft cabin, 3 staterooms, and if there is a lower helm, there is no side door access to the foredecks at all. In the 47 POC, personally, I'd prefer that boat without the lower helm as it is, in my humble opinion, fairly useless on a practical standpoint.
So that leaves you with a cockpit style boat. Options there are either a trawler with a cockpit like the marine trader 49, the defever 49, some grand banks, etc. etc. You have to ask yourself what your primary objective is on the boat....are you wanting a slow moving trawler that won't much get over 10 knots and enjoy great fuel economy, or do you want a boat that maybe has semi-decent economy at that speed but still allows you to go 20 knots or more when you feel like burning through a tank of diesel. You may want to give a serious look at the mid/late 1980's Bayliner 4588 or the newer 1995 and up Bayliner 4788's. They get decent fuel burn at hull speed, will get up and go if you want to do that, have a cockpit suitable for diving, etc., a large bridge with a davit for a dinghy, and a great pilothouse lowerhelm.
Do you want to go fast or slow? Fuel economy important or not, Twin engine or single? Lots of factors.