I always recommend liveaboards over all other kinds of diving in Thailand. It is by far the most enjoyable and relaxing way to dive there. There isn't much shore diving, so the choice is between daytrips and liveaboards. I have been on about 10 liveaboard trips on MV Giamani, MV Manta Queen, MV Scuba Adventure, and MV Pawara. I am going on another two consecutive trips next week. From what I understand the Thailand Agressor is not very good value compared to the other offerings in the area.
I have always felt quite safe in Thailand. Of course i don't leave valuables lying around but tuck them away in my luggage somewhere. Ironically, another guest on the Manta Queen lost all her travel documents because she handed them over to a crew member for safe keeping in the ship safe. He accidentally dropped it all into the sea!
April is the end of the season, so make sure that trips are available for your dates. The marine parks will close for low season.
You can check out StevenL's web site (see his signature above) for an overview of the available boats.
Have a great trip!