If we're speculating... probably it'll be similar to flying's "only areas with a strong wind aspect" handwave.
Specific areas of the ocean/lakes/etc will have a special elemental aspect that allows people to dive there without concern. This explains why sailors can still drown at sea, for example, and why swimming (and diving) are limited to specific locations.
Specific areas of the ocean/lakes/etc will have a special elemental aspect that allows people to dive there without concern. This explains why sailors can still drown at sea, for example, and why swimming (and diving) are limited to specific locations.