Ideally you always want to have access to a boat, just to appease Murphy's Law and make a crash less likely, but it is possible to manage without.
My club flies from a loch (in Scotland) just over a mile long.
We have access at both ends and I generally fly only with an onshore wind, anything from 3 to 10 mph is OK, and then you don't really need a boat. If the engine stops the model will drift in to shore. Even if you have a crash the bits come in eventually. Just don't pile it in hard a long way out or you'll have a long wait.

We have access to a boat at one end of our loch, but I last used it 10 years ago - and that was to pick up the wreckage of a landplane which stopped listening to the radio. When we run Splash-in events we naturally hire a boat specially for the occasion. See the gallery on our club website which includes photos and video clips of splash-ins at
www.gvmac.com