Try this very comprehensive website for ferries between islands. You can even pre-book your seats with them.
http://paleologos.forth-crs.gr/english/npgres.exe?PM=BR
Kath

thanks for that - found a similar one online but was saying stuff like ddepart at 6am arrive at 3am!!! so didnt trust it

this one seems more like it thank you so much

It is very easy there are 2 hydrofoils, 2 catamarans and 1 conventional ferry per day running on that route.
Hydrofoils roughly 08:00,18:00
Catamarans roughly 08:00,16:00
Conventional ferry 03:00-04:00
The conventional ferries and catamarans depart Kos from the main quay south side of harbour by the castle, the hydrofoils from a small quay 500 yds to the south.

I use 'Hermes' travel in Kos town for ferry tickets just behind the city bus station. Just buy them the day before or as soon as you arrive on Kos.

In Rhodes town the hydrofoils arrive/dep from Mandraki harbour, Catamarans from Kolona quay 500 yd south, and conventional ferries from the main ferry quay 500 yd further on.

Always check timings and departure points with the ticket agents.
The hydrofoils/cats are 2x speed and price of the conventional ferry.

ouzo - thank you for that information its sooooo helpful. will probably just wait and book once we arrive saying as theres 5 options a day - were sure to get on 1 of them.

THe Dodecanese Pride/Express catamaran is about the same price as the hydrofoil and for me, is much preferable (you can sit outside!) - neither is cheap - but they're quick and reliable - if you get one of the main line ferries, those generally are delayed!

Enjoy - I love the Dodecanese! Don't wish to confuse the issue, but just hopping up from Kos to Patmos and back has a lot to recommend it - the islands in between are all very different!

when you say neither is cheap - what rough cost you talking?

priced up a ferry online was somthing stupid like 15 euros each!!!! soooo cheap

http://www.12ne.gr/en/Routes/Fares.aspx 27.60 euros single! But it does only take 2.5 hours - the old DANE boats used to take 4 hours plus....