I ran an underwater adventure a few months back and it worked well.
First, know (and understand!!!) the rules for underwater combat (PHB p.198) and sufforcating (PHB p.183).
Second, magic items can help obviously. One PC had a ring of swimming while another had gloves of climbing and swimming. This meant these two had better swimming speeds (40 feet and 30 feet, respectively). The second was a rogue, so with Cunning Action could do another Dash action and move 90 feet in a round.
A cap of water breathing is an uncommon, minor tier item (no attunement, which the other two need), which works to breath underwater, but doesn't help your speed. Depending on how common magic is in your game world, finding or even questing to create a bunch of caps might not be too bad and could be a precursor to the adventure. The drawback to the cap is an enemy can attempt to remove it, thus denying the PC of its benefits!!! I have used the rules for disarming in such cases.
The simpliest item is the potion of water breathing, but that only lasts for 1 hour.
Spells, of course, the biggest IMO is water breathing. As a ritual and with 24 hour duration, you only need to cast it once a day. And it affects up to eight creatures IIRC so a single casting took care of the entire party. Alter self (2nd level) can work for a single person, but if the PCs are high enough level, water breathing is a must.
Finally, depending on the water, vision will be very limited! My adventure was in a murky, cloudy swamp-like water, so I treated the entire area lightly-obscured and limited vision from 30-60 feet (d4+2 x10 feet); it was heavily-obscured beyond this. I used a VTT with fog of war and lighting effects to create the feeling of being surrounded by haze and gloom. Because this was impediments in the water, not just lack of light, darkvision past the current limited vision didn't help. You simply couldn't see because of physical interruption to your sight, not just lack of light.
Hope that helps.