Hi michele!
I've done a fair amount of Puget Sound cruising on my P19 and my 5HP motor
has always seemed adequate. I think you're right in keeping weight down. On
both P15s and P19s, too much weight in the stern adversely affects
performance. I'd think a 3.5 or 4hp motor would be plenty for a P15, but
I'll let the P15 sailors here comment on that since they know the 15 far
better than I do.
I intend to buy a propellor with a little shallower pitch and use my current
one as a spare. A lot of 5hp motors are assumed to be intended for
lightweight rowboats and the prop they usually come with is designed to get
a light boat planing. I think a shallower pitch would get a little more
usable power from my 5hp pushing my relatively heavy p19.
Currents are a bear in Saratoga Passage. Definitely learn when and where the
currents are. It once took me over 6 hours motoring to get from Port Gardner
(everett) to Utsalady Bay (north end of camano) because I hit an ebb
current. On the the way back, i calculated the current more correctly and it
only took like 3.5 hours. Generally, tide rising means a NORTH current in
saratoga passage, and vice versa. Elsewhere in the sound, the opposite is
generally true. I don't think currents exceed 3 knots in the passage, but
you should double check.
I also found kelp beds to be a bit of a hassle cruising through there.