I have JL's in our Mastercraft boat and they are ok and in our 72 bronco I went with wet sounds in cans mounted to cage.
http:/marine-speaker-review.toptenreviews.com/
I have purchased the original kick pods and they have not been installed as yet as I was going to attempt to put in. The Polk 651s is a slim mount and you will not find much shallower. There are not huge cross section differences but in the kicks 3/4 of a inch with a cage maybe a factor.
Hope the link works...
The Polks are pretty much as thin as you will find and there are marine grade.