Overview
For a long time now, the only option to obtain any form of swim speed has been located in the pearl shop, under a 2,200 pearl price tag on a costume that locks to a single class. I'd like to suggest a loyalty or questline diving suit that may not be as potent as the pearl shop costumes, but a reasonable choice for people who wish not to spend $22 on a swimming costume, or those who want an alternative for their other characters.
Comparison and Reasonable Benefit

I personally believe that such a diving suit should be locked behind a short questline in Port Ratt, just like the Vadabin Diving Mask. For example, a Vadabin Diving Suit with a quest that should be exponentially harder than the quests to acquire the diving mask, for an exponentially better reward. The Pearl Shop Splat Fisher's and Da-Dum-Da-Dum Diving Suit boast a massive 149% Swim speed bonus alongside other benefits, and I don't believe that it is unreasonable to give the Vadabin Diving Suit half or even a third of their potency, as a 50% or 75% swimming speed bonus will still be largely beneficial and noticeable, especially for those who spend extended periods of time out at sea who don't own one of the Pearl Shop costumes.
Justification and Monetary Interference
I don't think Pearl Abyss would have to worry too much about this kind of addition interfering too much into their monetary gain as the Vadabin Diving Suit would still be far less potent, encouraging people who plan to spend more time on the sea to spend the money to get more of a bonus, while also providing "Free to Play" players some kind of tool to help them traverse the ocean.
Extra Detail, More Comparison
Below, I will put the costs and benefits of the Pearl Shop Splat Fisher's/Shark Suit, and next to them a concept of the costs bonuses a Vadabin Diving Suit and Mask Set may have.
Splat Fisher's Costume                        Da-Dum-Da-Dum Diving Suit Set                           Vadabin Diving Set
2,200 Pearls (22 USD)                            2,200 Pearls (22 USD)                                             Questline (Perhaps Sea Monster Materials)
+1 Fishing                                              Underwater Breathing +30s                                   Swimming Speed +50%
Swimming Speed +90%                          Swimming Speed +90%                                         +1 Movement Speed                  
[Less Swimming Stamina-                    [Less Swimming Stamina
Consumption]                                        Consumption]
Set Effect                                               Set Effect                                                                 Set Effect [Combined With Vadabin Mask]
Jump Height +80CM                             Swimming Speed +59%                                           Swimming Speed +25%
Swimming Speed +59%                                                                                                           Underwater Breathing +10s       
Ending Note

I believe that this is the appropriate time to add something like this to the game to encourage people to begin sailing out there and getting their equipment as the Great Expedition draws nearer, and the oceans of Black Desert become more populated. Otherwise, good luck and safe travels, Adventurers!