Magic mouths don't have a 170' detection range, it's 30 ft. Secondly, if you want them to scream, you have to scream bloody murder when you set the message. That surely won't attract attention. Thirdly, magic mouths have no special detection methods, nor even perception scores. They can only see things clearly seeable, so you can easily sneak by them (like an ambusher might).
Leomund's problem isn't that it's a ritual, it's that Leomund's Hut is one of the more powerful spells in the game. Just straight out casting it is a good use of resources.
Heh, good luck, they eat anything. A metal boot's not even a speedbump to a shark.
How do you know it's coming? Leomund's Secret Weather Satellite(tm)? Heavy seas from storms can often precede storms by days for really powerful storms. I'm guessing you haven't spent much time at sea. 10' swells would wreak havoc on any attempt to walk across a sea, are not uncommon near storms (not even powerful ones, hurricanes can generate sustained swells of 50+' quite far from the storm), and can last for days depending on weather.
Good luck getting your cargo to the bottom. Boxes that are watertight tend to float.
Also, good luck avoiding sharks now that your clever metal boots are underwater.
Firstly, you have a 9th level caster around, and that's how you utlize his time. Okay. But what you have there is that caster spending 10 mins for each group of 8 soldiers. That's an hour for 48 guys, which can then only telepathically speak to each other within their 8 man groups -- not across all the others. Each casting is a separate group. And then you have to start all over again to get keep those same 48 guys in their telepathic 8 man groups. Oh, and they all have to be present for the entire casting, so they can stand guard for like 30-40 minutes in their cool telepathic state before it's time to rotate back to the ritual spot.
Surely the enemy is aware of this, and just hangs out a few days, wasting your 9th level casters time and having a good laugh.