A mobile sling would work for short distances, but for a long-time equipment, you'd also need to compensate for different weight in atmosphere vs hydrosphere (i.e. make sure the occupant doesnt suffocate under their own weight).

There would also be a few other oddball issues - the two I can think of are sun exposure and keeping skin moist, but they're kind of easier to solve? Maybe?

This would also be affected by weight. A porpoise or dolphin would be uncomplicated and could likely enter most human buildings.

Larger whales? They'd need supergizmology to get on land, and in fact it might be easier to use one of those robot doppelganger thingamajigs to travel in human buildings.