Slimjim,
If there are drains in the boxes my guess is they have hoses that connect to thru-hull fittings on either side and exit out the side of the hull just above the water line and exit there. As for water getting in, if you do not have a pump that pumps water in then you probably have to plug the drains and use a bucket to put water in there. I personally wouldn't want to put water in a forward compartment because that is the bounciest part of the boat.
If you are concerned that the compartments are not big enough for whatever species you target, you might consider getting an insulated "fish bag" to hold the bigger ones. That is what I did, although it seems to have cursed me because I haven't caught one big enough to put it to use yet.
Or if you have a need for a livewell or bait tank I would recommend making or buying one instead of using those forward compartments.
Sorry, I'm not very familiar with the 18's but thought I'd chime in.
Blue