astalakian cargo capsules.

The astalakians (crab people) developed a unique way of transporting bulk goods that was eventually adopted by merfolk and ojiongo (cephalopod people) once they realized how useful it was.
There are variations, but the basic principle is the same. The main component is a very large net, woven tight enough to encase whatever cargo is inside. Usually these end up being a container full of containers, so the net doesn't have to be very tight. The cargo is bundled into the net along with tightly woven balls with pockets of air to make the overall load neutrally buoyant. Then the capsule is tied with a series of rope to a large team of astalakian porters or a team of large crustaceans playing the role of a surface world oxen, donkey, or draft horse, or possible a single large sea creature like a colossal crab.
These processions are slow, but they can carry very heavy loads fairly economically. The few Scarterrans who have seen these (with magic to let them survive underwater of course), liken these capsules to river barges that Scarterrans use.
The astalakians and others have tamed or domesticated various sea creatures to act as pack animals. This includes some giant crabs that have been selectively bred to be so docile and slow witted that they can be led around with a fragrant bit of food like dangling a carrot in front of a hungry donkey.

Variants

The astalakians created the first capsules for moving large bulk cargos using pack animals they carefully trained.
Since beast magic is relatively common in Scaraqua, many choose to magically control sea creatures and forgo the need for full training of a beast of burden. This is fairly popular with karakhai (sharkmen) and more free-ranging merfolk.
Merfolk have developed much smaller capsules that use the same basic principles to let armies on the march (can you march while swimming?) drag their personal kit with them. This is very popular with legionaires of the Oshamni Empire.
Some merfolk have created capsules that float entirely, bobbing on the surface of the water while they dragged by shallow swimming fish wearing harneses, either trained or magically controlled. This is a little bit faster than conventional astalakian capsules but they run the risk of being spotted by curious Scarterran sailors sailing on the surface. Thus, this is usually done in remote waters far from shipping lanes though some daring merfolk have moved these loads near Scarterran ships confusing the sailors immensely.