Like all Bethesdas open world games underwater exploration has always been a very cool feature.
Some titles like Skyrim and Oblivion touch on it rather briefly while Morrowind featured entire cells and caves (grottos) of deep murky water that held various treasures and perils as well as entire daedric temples that had collapsed into the sea...
The Fallout 4 concept art features many visual references to water, ships, and even submarines being a larger theme than in the previous desert filled expanses. This video by The Pre-War Hub has gone into a lot of detail on what we can perhaps expect to see...
Underwater exploration in any rpg is always incredibly cool and in games like Bioshock 2 provides the player with some awe inspiring visuals. While underwater cities are probably unrealistic in Fallout 4 its location and setting is practically begging for us to take a dip in a radiation suit and find out what's going on beneath the waves...
...with high radiation levels, Mirelurks and mutations its bound not to be good news! But that's definitely part of the fun and I hope its been implemented well!
What about Sonic! Lol...
I only like underwater exploration when there's good rewards to be found. In Morrowind exploring water caves often led to amazing loot and because you could find daedric items at level 1 it often made the risk of drowning worth it.
I agree that they don't seem to have got it right since, underwater locations haven't been given enough attention to make them worth the drag of not being able to swing a weapon and the movement speed decrease just makes it even more painful...