Hi CathyH and other P3D folks,
P3D doesn't have a "FX_wake_coastguard.fx". So, if you're installing and running that file in P3D, it is an FSX file.
The vehicle in question (in . . . simobjects/boats/veh_ . . ) has a "sim.cfg" file, just like an aircraft.cfg file. In it is an [effects] section like so:
[EFFECTS]
//wake = fx_wake_coastguard
wake = fx_wake_s (change last part to "s" or "ss", or "m", or "L", or "carrier" instead of "coastguard")
Open the "Sim.cfg" file with notepad. You need to change (or disable with //) the line for wake as in the example above.
See if that helps.
(I haven't tried this myself, I just thought of it. So here's hoping . . .)