3 things - the height of the transmitter.
Generally, the higher the transmitter above the water, the better. Surface interference is reduced.
You are still stuck with line of sight.
While the power of the transmiter is limited legally in most parts of the world, range can be improved by introducing the right kind of second receiver to improve sensitivity without compromising selectivity. Does that count as 4 things?. Just something I read about, but the extension receiver cuts the error rate at low power (i.e. long range). Liked by the flyboys. As range increases, the received power reduces and drops down into the background noise level, the extra receiver gathers more signal and between them they sort out a valid signal at greater range. Since this is high-end magic, I expect it will be in Futaba's book.
My bottom end radios work very well at ranges beyond where I can easily see what if happening ith the boat - 250 yards plus - so extreme range is not a problem for me.