If you hop on a boat to find one you basically just have to keep your eyes peeled and get lucky. However, there is a neat little way of (sometimes) finding them. There is a type of villager called the "cartographer" that sells treasure maps. Treasure maps show you one point of interest in your world (including a water temple, but it could also be a jungle temple, and such). Try finding this villager and buying a lot of maps from him.
If you have uncovered your entire map and not found one then there isn't one to find. There is no guarentee that you will get a monument from a random seed. The 'Seeds' forum in the 'Minecraft: Editions' section can point you in the right direction if your out of luck on your current world.