If you're specifically looking for liveaboards, sites like liveaboard.com will give you plenty of options.
The closest to Penang would probably be the Similans in Thailand. There are direct flights to Phuket (where most of the liveaboards operate from) from Penang. April is the tail end of the season for Similans liveaboards but should still be plenty of diving. Large selection of boats to choose from, from low end to high end. Lots of the liveaboard operators frequent this board and other scubaboard members can help you book.
If you are there in late April or later, I would suggest the east coast of peninsular Malaysia. Places like Redang Island, Perhentian, Tioman, etc. Easy diving with a short flight from Penang. Don't recommend in early April though, as the monsoons may not have died down and the waters could be rough and murky.
