To put it bluntly, Crashing may be a god, but that means bugger all when the force of an army will be battering him like crap. The greatest advantage the sea horde has is Crashing and the Leviathans, but Crashing can only be in one place at a time, and the Leviathans are so grotesquely large that they can be easily hit by artillery and those moving statues M&S has and killed.
As for the horde itself, it is immediately put at a disadvantage by unfamiliar terrain (they haven't stepped on land in years doubtlessly), inexperience, an archaic understanding of war itself compared to the clear and definite modern understanding moon and star has, but most of all their own weaponry. Thanks to living in the ocean, they will have no ranged combat units, because if you hadn't noticed bows don't work under water and they would have no way to practice archery in the ocean, this wouldn't be so bad if they were like the Stormwing and could fly, but they can't, and so they consist of a lumbering horde dependent on melee combat against a foe with ranged superiority. Technologically they are not on the save level of war Technology as M&S either, so thats another issue they will face, and tactically they will be terrible due to being at peace for so long. In an ocean war they stand to win that conflict, but on a land war they will be completely overwhelmed, the only way they could win is if they accumulate in one horde behind Crashing and stick together, but thats just asking to be ambushed and anihiliated, furthermore, the more his forces move inland the more likely their supply lines are to be cut off, starvation is just as useful a tool as any in war.
Furthermore, and most definitely so, Moon and Star are prepared for them, Clashing has been causing a big stir up, and due to his attacks on the slave pens next to Canterlot the army will have been likely pulled back in response to it and placed on high alert. Now they have landed with a fuck huge beastie and any local farmer or fisherpony could have spotted them camping nicely where they are and reported it to the authorities.
In other words, if we took this realistically, they would be completely fucked.