Are sharks the only aquatic threats in this universe? Just asking because I thought I remembered seeing the stat blocks for some other creatures that live in the oceans.
The biggest danger would be pirates and unscrupulous business competitors. All they need to do is to wait until the water walkers are more than 30 minutes away from shore and then show up with a ship and a few archers. As soon as the ritual casting starts (because Water Walking and Tensor's Floating Disk only last an hour, so it needs to be cast regularly) and they start firing arrows at that point. Once the wizard is dead (or has his ritual disrupted enough to fail and he sinks under the water, they have their own wizard cast waterbreathing on some of their crew who dives in and retrieves the cargo.
A little mathematics...
Tensor's Floating Disk capacity is 500lb maximum. However, you wouldn't want to load it up to that level because if you did and a seagull landed on it, the total weight would be over 500lb immediately ending the spell. To be safe, the maximum effective capacity is probably only 450lb.
Also, as both it and Water Walking needs to be cast every hour, the wizard casting the Water Walking shouldn't be the one casting Tensor's floating disk. That means the maximum capacity would be for a party of 10 wizards: 1 5th level to cast Water Walking and 9 1st level to cast Tensor's Floating Disk. That gives an effective carrying capacity of 9x450lb = 4050lb. That's not really a lot when compared to the carrying capacity of a ship.
However, a party of 10 wizards (9 of which are 1st level) would be really vulnerable to pirates, so some of the water walkers would need to be guards (especially for that 10 minutes every hour when the wizards are all casting rituals), which further reduces the carrying capacity.
Finally, a ship can stay out overnight. The water walkers would need to find dry land or do a forced march, because otherwise they'll drown when they stop for the night.
Just as an extra, the PHB indicates that casting a simple 1st level spell like Cure Wounds, a spellcaster would charge 10-50gp. Let's say 10gp. For one eight hour day, that's eight castings x 10 wizards (the 5th level wizard is obviously a part owner of the venture to only be charging 10gp per cast of a 3rd level spell) which equals 800gp per day. It wouldn't take long before the economics makes ship travel that much cheaper (greater carrying capacity, better range, better safety, and a much lower daily cost after the initial upfront cost of purchasing the ship).
Economically, water walkers with Tensor's Floating Disk would only work in a few specialised areas. Most of the time, a ship is a much better option.
Now, onto the Magic Mouth case. Essentially, this is setting up an array of not-particularly bright sentries, and isn't a bad idea apart form the cost (110gp per night -- I really hope adventuring pays well!).
However, the premise is that this is a world where ritual magic is used, so that means the bandits can have their 1st level cleric/wizard use Detect Magic to locate those Magic Mouths. The arrangement has almost no overlap between the magic mouths so that leaves a couple of options for the bandits.
Option 1: Have a 5th level wizard dispel one of the magic mouths. That opens a hole in the wall (effectively 'killing' one of the sentries) and allowing the bandits to sneak in.
Option 2: Magic mouth is triggered by visual or auditory information. That means it's vulnerable to things like invisibility or stealth...especially in a woods where visibility could be a problem. The spell doesn't say one way or another, but if I were the GM I would rule that a 2nd level spell used normal perception (probably the Wizard's passive perception) to detect anyone trying to sneak past.
If Leomund's Tiny Hut is used, then okay, the party is still safe, but that's a spell that generally only cast once a day, which means that making it a ritual saves one spell slot a day. That's not particularly powerful. Decent and worthwhile, but not a game-changer.
In short, Leomund's Tiny Hut is a great spell, but using a circle of Magic Mouths as magical sentries is not without its flaws. If the technique is known, the countermeasures listed above would also be known, and you can expect bandits to use them.
Finally, for Unseen Servant, it doesn't trigger every trap. Indeed, I would argue that it triggers very few unless the wizard instructs it appropriately.
Pressure plates? It's an invisible, shapeless, mindless force. Does it exert enough force on the ground to trigger a pressure plate? Probably not unless the wizard instructs it accordingly. Also, supposing a 10' wide corridor, the wizard would have to instruct it cover every square foot of that corridor...at 15' per turn. Assuming it had to go up one side and back down the other, that's two turns before the party moves forward 15' (taking another turn because that's the movement rate of the Unseen Servant), so that means it takes 3 turns to move 15'...
Trip wires? Probably triggered, so that's an easy one.
Visually trigger traps (eg. the goblin with his hand on a lever to trigger a pit trap as soon as he sees someone)? Definitely not triggered by an Unseen Servant.
Magical traps (such as Magic Mouths to yell a warning)? Again, not necessarily triggered by an Unseen Servant.
And, finally, Detect Magic. Please note that Nystul's Magic Aura is a 2nd level spell and if you cast it the same way for 30 days in a row, it lasts until dispelled. You want that magical trap hidden from Detect Magic? Not a problem! 30 days of casting a 2nd level spell and the party that relies on Detect Magic just ran into whatever trap you wanted
Rituals are decent. They're not overpowered.
