Deep Water Aqueducts
Rapid, long distance travel within the sea is often handled by creating tunnels of low water pressure via a Deep Water Aqueduct spell. The shape of the tunnel is similar to a cone starting at the caster’s hand. As the cone extends, it displaces large amounts of water, allowing the caster to propel themselves forward with significantly less energy (thereby making the swim easier, faster, or a bit of both).
Corkscrew groove-shaped channels in the cone allow safe passage for any creatures caught in the way of the Aqueduct, though inexperienced merwitches often have trouble directing traffic properly (the result is either hilarious or incredibly indigent, depending on which side of the cone you are on.)
Mermaids can survive (and regularly travel) in opposing water bodies, but at a significantly slower rate. Freshwater mermaids tend to be smaller and more nimble than their sea-faring cousins, who require greater size and strength to survive the vast expanses of the ocean. Aqueducts alter the waterscape nicely, allowing smooth swimming. Conversely, ocean mermaids often have trouble in relatively shallow rivers and lakes. Creating an Aqueduct, then collapsing the focal area to a point around their gills, allow them to breathe regularly.