how often does the water launch run? What are the scheduled crossings?
As I remember, there was one around 9am and another around 11am and another after 1pm then coming back there were 3 or 4 after noon up til about 8pm. But you should write them from their website and they will send you a schedule. The frequency and lateness of trips somewhat depends on the season as I recall. It is a very sweet boat with inside and outside seating for about 10 people and a lovely way to travel to Karikoy where you can get the tram to the old city (Sultanhamet) or ferries to Kadikoy. You need to schedule in advance with the front desk especially during the high tourist season. We were there in April so it wasn't a problem. Hope that is helpful.
They run about 4-5 times in the morning and the same in the evening. HOWEVER, you must let the reception desk know which of the times you want to take at least two hours before, I believe (for both there and back). Moreover, the water launch in the evening goes from two times at the same place it drops you off in the morning and then the last 2/3 times at another place further away where there are many clubs/restaurants.
Get an answer from Sumahan on the Water staff and past guests.