Thetis' Blessing is a lvl 9 Enchantment ritual that allows everyone to enter the water. If you prepare some armies on your new second front, this will catch the enemy totally unaware.
And if you want to close your second front to prevent counterattacks, just kill the caster. (I don't know what happens to non water breathers in a water nation in such a case, but I think it would be biblical
).
Against a water nation I would just cast the lvl 7 Alteration ritual, sea of ice. (first ask your allies/foes for water gems to overcast this. Bye bye water nation (hello new archenemy
)).