On a direct drive RC boat, a 380 motor can produce much higher RPM. However, it can get overheated pretty easily even with water cooling. 390 is slower but it produces more torque, which may be necessary for pushing a larger propeller.
Simply put, if you want more speed and motor lifespan is not a concern, go for a 380. Otherwise, a 390 will definitely last longer (due to less heat).
FYI, RS390 is longer than a 380.
In fact, there is a major problem with the shorter length of a 380 – the entire casing is covered by the cooling coil blocking the venting holes, making it very difficult for air circulation to take place (this is why a 380 can get overheated easily):
To allow a 380 to “breath” properly, you need to use a smaller cooling coil:
If you want your 380 to live longer, let it work with a smaller propeller (less workload that is). But I can tell you that your 380 will still fail eventually, just that it may last slightly longer…
IMHO, a 390 would be much better. Below shows a perfect configuration:
I am a bit skeptical regarding 395 motor. Most if not all 395 motors offer pretty low RPM while requiring pretty high voltage, which is not ideal for small RC boats.
You want to know that 390 motor is now quite common on RC car too. The performance is in fact quite good: