So if you attack a PT boat in 'fighter mode' and get hammered, that doesn't affect your K/D, even though you got K'ed, it's not a D? Feasibly I could up a Hurricane with the 40mm, and die fifty times killing PT boats and the like, and it isn't going to affect my K/D? What's up with that? 
-Twitchy
Killing PT boats (or GV's) will not effect your K/D for
fighter mode. Only kills against other airplanes count for you while in
fighter mode.
But any time you die it counts against you, and it doesn't matter what the reason is. Crash, get shot, fall off the CV, whatever... Even ditching counts as a death in the scoring system.
Actually, even shooting at PT boats (or GV's, or even buildings, ack guns, or other objects) will "hurt" your fighter score, because only bullets that hit other
planes count as hits
in fighter mode. Even hitting a GV, PT boat, or object will count as a "miss". So, whether you kill what you're shooting at or not, those rounds count as misses and will
lower your hit %.
If you want a "good" fighter score, you need to kill other planes as quickly as you can, without dying, and without missing, and land as quickly as possible. You don't want to waste much time flying around or RTBing, because it will hurt your kills/time. You don't want to die, because it hurts your K/D. You don't want to miss what your shooting at, because it hurts your hit% and your kills/time (because you'll need to rtb for more ammo). You also don't want to end your sortie without several kills, because it will hurt your kills/sortie.
If you get enough kills, it may even be preferable to die rather than hurt your kills/time by RTBing, believe it or not.
If you can get your K/D up to around 7-8 to 1, your K/S up to around 3 to 1, your hit% above 10%, and your kills/time up to about 8, you'll probably rank in the top 20 in fighter mode.
If you want to get credit for hits against and kills of PT's, GV's, and ground objects, fly in Attack Mode...