I have worked 10 watt
backpack stations in the UK when they were by the sea side. No hope to
do
so when they are inland.
Three or four weeks ago I drove down to the beach to do
some fishing, bbqing, and general r&r. Of course, I had my
radio. I pulled my truck right up next to the water. Didn't
bother throwing a "water" ground wire out. Just ran normal
with my "extended" 14 ft mobile whip. The coil 8 ft above the
base of the whip. 100w. Was yakking on 40m late at night, and among
all the stations, mostly home stations, some running power,
I was one of the loudest ones on the freq. Can't beat a good
sal****er location.

Also, a mobile late at night on 40m,
generally is as good as most normal height dipoles, etc. In fact,
I had one guy do a test. He was in the house in FLA, and I wagered
that his signal would increase if he went to his mobile.
He did, and it did. MK