If your actual goal is to be a pirate for a while ...
... per your comment under the question ...
Seriously? . . . I just want to be a pirate. We don't have the time to take 250 days. In the old days (2e) it was accepted by many that if you gained 1000 xp using the skill, you could become proficient. I haven't seen anything like that.
... then all you need to do is to hire an NPC who has proficiency with Vehicles(Water) and have them drive the ship while the rest of you conduct yourselves in as piratical a manner as you like for as long as you like.
- The expanded rules for ships in Ghosts of Saltmarsh include actions that characters who lack a proficiency in Water Vehicles can use to assist in the running of a ship (p. 198-199).
- As usual, any task or action can be presented as a non-proficient ability check based on an ability score: work with your DM to sort out the practical details for these efforts. (Examples: Repair? Strength check. Splicing a line? Dexterity check, etc).
If you don't intend to be doing piracy for at least 250 in-game days, you can still do any number of in-character things with the ship.
- Promise this NPC (as an incentive) that the ship is his or hers when you stop pirating. That's a huge incentive to retain this NPC's loyalty.
- Sell the ship at your last port of call when you tire of pirating. (Prices in PHB, p. 157)
- Trade/barter the ship for something valuable when you tire of pirating. (Prices in PHB, p. 157)
- About pricing: sailing ships go for 10,000 GP, Galleys for 30,000 GP, Longships for 10,000 GP.
- Use the ship as a decoy for a monster or an opposing force, and set it ablaze in a last act of piracy / randomness / cleverness when you are done with pirating.
- Other things that you dream up, too many to count. You've enough timber to build a few houses ...
Leave your options open - you may fall in love with piracy
If, as the adventure runs its course, you end up being a pirate for longer than 250 in-game days (it's hard to predict how an adventuring life plays out once you become a pirate) you may grow to love it and do it for longer than you currently envision. Whether this plays out or not, I still suggest that you work with your GM now, ahead of time, to accrue that proficiency / training until your pirating career does finally end.
- Experience based point here: many years ago, my players ended up
beginning their seaborne career after capturing a trading ship during
a battle. The campaign took a decidedly maritime turn at that point,
and they sailed all over the game world before finally losing the
ship to a fire during a battle in a harbor. The game didn't start out
at sea, that's how things worked out. Just over one in-game year spent
both as "merchants" and as "privateers" for various nobles they met. Granted, our campaign was a bit "sand box" in style, and yours may not be. The point here is: once you start engaging in a maritime campaign, you may find that you really like piracy.
It taking a long time to become proficient with water vehicles is pretty consistent with how the Sailor Background offers that proficiency - you spent your whole life to date sailing the seas. It also offers a bit of verisimilitude regarding what it takes to sail effectively. It's not like driving a car. I learned to sail some decades ago, but it was a hobby I could not then afford ... and then the kids came ... then 35 years later I was sailing on Lake Travis with a bunch of hung over friends on New Year's Day, and nearly rolled them into the water by being a bit rough on the helm.