On hammer testing:
* In the water is doubtful. Generally you are listening, not making dents.
* Not a chipping hammer or rubber hammer; must be a moderate ball peen hammer or equivalent (weight depending on thickness of steel). You need the hard surface. You are listening to the ring, not making holes.
* You can hammer test without damaging paint, but it requires patience and a lighter touch. A very hard plastic face hammer can work.
On UT testing:
* Yes, good gauges can read through good paint. However, bottom paint
is not generally "good paint." It is too soft and too poorly adhered, at least on old boats. You may get a good signal, you may not. If the gauge shows the A-scan blip this will be clear, so it will not be a guess. UT on the topsides through paint should be no problem.
* Yup, that is always the best bet. Eyes top tech for judging localized corrosion
On thick coating and fiberglass
* You've GOT to look underneath. Sprayed-on foam insulation
has been the death of many tanks
You got a look of good advise in these posts.