[Finalgenesis]

I find that where possible sticking to coastal provinces yields the best result for Ryleth.

Players are more wary of waging war on you as your raiders from the sea can be very annoying, and logistically it's much easier to move troops between coast and Sea. Coastal province are where you can project your sea power. Whereas if you have large inland provinces the logistic becomes difficult and you become a juicier target to land nations, not to mention you can't build your core national mage/troops inland (and limited ones for coastal forts, star child + hybrids).

Unless you are playing UGWIM mod, most land nation have great difficulty attacking you in water before late game and are forced to invest heavily into such an effort, combined with no UW PD for most land nation means that they have to either invest in a lot of actual troop garrison in water or risk being raided with impunity (scouts, starchild, teleporting starspawns). To summarize, attacks into water against a settled water nation takes immense effort, most land nation can't commit to (and any such invasion will likely fail unless they are much much stronger) this during early-mid game as they have to contend with other land nations, so as long as you don't dangle too big a fruit (say a clump of inland province) and stick to where you can project and threaten with your viking tactics, you should be okay.

I'll also say if land nation do attack your land holding in force, it's usually better not to meet toe to toe unless you are clearly superior, it's much more sensible to go viking/guerrilla style and take full advantage of your sea nation status, your army lineup don't compare favorably on land to most land nations and require heavy magic/summon/thugging backup. Poke them endlessly from the water where they can't feasibly touch you, combined with good diplomacy will take you a long way. Think on Baalz's analogy, you're a submarine, they are battleships (land nations).