Boas will soak, but it isn't strictly necessary. I'd give him a large bowl, but one that doesn't take up all the space in that tank. Ceramic is best - boas like to spill their water....
This! Though I found the best way to stop spills was to use a shallow plastic tub with a large footprint, and fill it up only 1/3-1/2 of the way. It allows them to soak if they want and even my 7' BCI girl hasn't dumped hers, and she's notorious for redecorating...