e boats would be a different varient but good , but u boats better , x craft would be interesting , we could use them to attack ports , and the cv group that are permentley in port and dorment, these could be targeted , put a charge under the hull ,destroy which ever ship necessery , and next time if that side still has the port they loose that particular vessel when it respawns, and the depth of the x craft could be fixed if you dive to deep you would die .