Ferry service opens 1 hour before the Magic Kingdom opens, and shuts down 90 minutes after park close.
We had lake view at Poly in early December, and could hear the horns.
Our strategy was to treat the horns as a "Feature" rather than a "bug". For one thing, it is really convenient to walk from Moorea to the TTC (Transportation and Ticket Center) where you catch the boat, and it is a quick ride to the MK. After doing this on the first night, we had a good idea what the ferry was, and the sounds just kind of faded into the background.
In the mornings, we sort of used it as an indicator that it was 7AM and time to think about getting up.