I have only had one incident that actually required pumping the boat. A partner fell out of his boat so I did an assisted rescue. We did a quick boat dump to get excess water and flipped the boat upright. We had perhaps two inches of water in the boat by the time he was able to get in. This was a cold water rescue, so by the time he was back in the boat, it was more critical to get him to shore and in warm/dry clothing than to pump out the little bit of water that was in the boat.

I guess my feeling is that if the water is rough enough to flood your boat, you probably are not going to be able to handle a pump without rafting up anyways. It is such an occasional event, that I'm not sure I would want to use space under the deck for this type of rigging. I usually carry my pump on the deck behind the cockpit.

Just my two cents.