I think the bit about zombie blood being more like gel than liquid was probably supposed to be how they were able to function at that depth-not the *deep* bottom, but at the crush depth of a Chinese nuclear sub IIRC-as to how they got there I think it had something to do with freighters overloaded with thousands of smuggled refugees dumping everyone into the drink after an outbreak. It's not shown to be much of a direct threat, more of an ongoing nuisance in the clean-up period with ghouls occasionally washing up on coast lines or just walking out of the surf. Either way, swimming in the ocean, seas or lakes would no longer hold much appeal.
i think that indeed is how zombies could cross oceans or wash ashore.
Assuming that the initial zombification started suddenly, all over the world. If you're on a Carnival cruise, and justhappen to be in the middle of the ocean (or at leats in between islands), it's possible that a boatload (literally) of people get infected. And as it's a new thing, people who have gotten bitten may jump overboard to get away, not knowing that they'll get zombified anyway.
And certainly issues like smuggled people will add to it as well.