I have now started planning my trip to Warsaw in July and I thought that I'll post some more details as I'll unearth them (and possibly a few follow up questions).
Firstly the sailing :
WTL operates several kinds of boats:
1) Boat to Serock.
Serock is a village on the shores of the Zegrze (Segrzynski) lake.
It's a once a day cruise on Friday, Saturday, Sunday in summer.
From 9.30 to 17.45 with a 2 hours stop in Serock.
More info :
http://www.ztm.waw.pl/?c=148&l=2
There is no mentioning of adequate or not adequate water levels this year.
It's a very long cruise in my opinion, probably too long for my liking unless
Serock is really worth a visit - is it?
2) Water tram cruise in Warsaw itself.
Operating Friday, Saturday, Sunday in summer.
~ 1 1/2 hours duration.
3 departures on Fridays, 4 on Saturdays, Sundays.
More info :
http://www.ztm.waw.pl/?c=147&l=2
3) Ferries across the river.
Every day.
More info :
http://www.ztm.waw.pl/?c=149&l=2
And prices for all 3 :
http://www.ztm.waw.pl/?c=170&l=2