An entertaining over-the-top war story. "The Sea Guerrillas" are the 7-man crew of an American PT Boat, posing as a Portuguese fishing vessel with a mission to attack and destroy German U-Boats.
In this fictionalized version of the war, U-Boats are using waters near Portugal as a safe haven (not so in real life) and are wreaking havoc on Allied shipping. The disguised PT boat is an Allied gambit to counter this. There are hundreds of fishing vessels in the water, so surfaced U-Boats generally ignored them. The PT boat is therefore able to get close before revealing the hidden weapons and depth charges they carry, then attack the submarine.
The action scenes are fast-paced and exciting. During these scenes, Ballenger shifts POV quickly between the Americans and the Germans. Without slowing the pace, he names many of the Germans and gives us a quick, effective sense of their personalities. Some are hard-core Nazis and it's an unrepentant pleasure to see them taken out. But others are just guys--in many ways no different from the American sailors. It gives this otherwise pure-action novel a sense of depth and allows a little bit of musing about the tragic nature of war without getting in the way of its main purpose.
And its main purpose is blowing up U-Boats. After taking out a number of solitary subs, the PT boat must attack a wolf pack. This ends with a damaged U-Boat chasing the damaged and ammo-depleted PT boat back to its hidden base, where the Americans must come up with an innovative tactic for taking out this last sub before it gets them.