Depending on the map, it can be enormously tactically useful. Remember, fliers can't cross water at all, so sailors can sometimes move farther even than fliers. The best of course is an amphibious flying sailor. They have move options that make my head hurt.
And, the way I picture it, by "large lake" you mean something on the order of the Black Sea. That's not too bad for one month with an army...
But you're right, it's not a hugely impressive ability. Vastnesses can move farther.
