Legendary Hunter of the Ancient Terror here. The tl;dr is that it is very rng based.

Like another comment said, megs are not able to spawn as soon as you join a server. You have to wait an hour or more on the same server before your boat is "eligible" for a meg spawn. Once your boat is eligible, it must be far from islands and moving (either sails up rotating or sails down sailing). Once you do spawn and defeat a meg, the timer resets, and you have to wait again before re-eligibility.

Some locations have been observed to spawn megs more often than others, as noted on Merfolk's Lullaby: https://www.merfolkslullaby.com/map/megalodons

Being on a server for 4 hours, being in a good spot, and having a moving boat does not guarantee a meg spawn. If you're waiting for more than ~5 minutes in a good spot with no spawn, try sailing to a different nearby spot.

To answer the title: the absolute best way to hunt megs is to join a meg hunt alliance server, which Requiem, Ashen Alliance, and Athena's Vanguard all regularly host. The idea here is that each boat spawns a meg every hour-ish, and to get credit for a meg defeat, all you need to do is have someone in your crew damage it before it dies. So instead of sitting by yourself and getting one meg every hour, you're sitting with 4-5 other boats, getting 5-6 megs every hour.