Ferry Hiroshima to Miyajima

How long does the trip take and what are the usual sea conditions rough/smooth ? What are the schedules?

http://www.japan-guide.com/e/e3454.html

If you use the tram, it goes right past Peace Park and the Atomic Dome. You can just see it as you go over the bridge or you can stop there.

How long does the trip take and what are the usual sea conditions rough/smooth ? What are the schedules?

Departing from Miyajima-guchi ferry takes about 10 minutes and Sea condition in general is smooth

Good information here :

http://miyajima-matsudai.co.jp/english/fares-timetable/index.html

There are two competing ferry companies.

JR West Miyajima Ferry operates four ferries an hour from around 8:00 to around 18:30 in each of the directions, less frequently in earlier and later hours. Its ferries departing from Miyajimaguchi Pier at 09:10 to 16:10 take a route to approach Grand Torii gate of Itsukushima Shrine.
http://www.jr-miyajimaferry.co.jp/time/
: Departure times for Miyajimaguchi Pier are on the left.
: Departure times for Miyajima Pier are on the right.

Miyajima Matsudai Tourist Ship, which is mentioned above by liem, is a subsidiary company of Hiroshima Electric Railway [Hiroden]. Its Miyajimaguchi Pier is located just near Hiroden-miyajima-guchi stop on Line 2 of Hiroden streetcar.

From Miyajimaguchi Station of JR-West, you can proceed on the left or right side of the road leading to the ferry port, partly via an underground pathway. The pier for JR West Ferry will be in front of you, and the pier for Matsudai Ferry will be to your left.

Are you talking about the ferries that go direct to Miyajima from the Hiroshima Peace Park. Those take about an hour and cost 2000 yen one way. More info here:

http://www.japan-guide.com/e/e3454.html