Mfcttrf

Our party inadvertently captured a ship.

Is there any way to gain a proficiency now that we're 6th level? If I have to wait two more levels to get the Skilled Feat, I'm afraid we'll end up sinking her.

If I spend time on board, under the guidance of a skilled captain, is it really going to take 250 days?

Use downtime training rules from Xanathar's Guide to Everything

The standard Training downtime option given in the PHB does indeed require a significant time investment - so significant that it's not a practical option in many campaigns. Luckily, however, the Downtime Revisited chapter in Xanathar's Guide to Everything offers alternative rules for training that require much less time:

Receiving training in a language or tool typically takes at least ten workweeks, but this time is reduced by a number of workweeks equal to the character’s Intelligence modifier (an Intelligence penalty doesn’t increase the time needed). Training costs 25 gp per workweek.

Under these rules, assuming your DM doesn't decide that proficiency with water vehicles is more difficult to learn than standard, an exceptionally smart character might be able to pick up the knack of sailing after just five weeks with an appropriate tutor, but even a dunce can learn the basics with ten weeks of tutelage.

It would seem appropriate for that to take the form of "on the job" training in the case of learning to sail, so you could continue your normal activities and treat all the days you'll inevitably be spending travelling around as downtime to learn the skills - though I would also rule that you'd need still need to have a properly trained crew to handle the ship effectively while you're getting your sea legs, and you will need to pay someone specifically to tutor you.

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 a number of actions that characters who lack a proficiency in Water Vehicles can participate in to assist in the running of a ship(p. 198, 199). As usual, any attempted task or action can be presented as a non-proficient ability check based on an ability score: work with your DM to arrive at the practical details regarding these contributions. (Examples: repair? Srength check. Splicing a line? Dexterity check, etc).

If you don't intend to be doing piracy for at least 250 game days, then you can do any number of in-character things with the ship.

1. 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.


2. Sell the ship at your last port of call when you tire of pirating. (Prices in PHB, p. 157)


3. 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.
   
   


4. 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.


5. 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 days (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 imagine. 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 during a battle in a harbor. (Fire). The game didn't start out
  at sea, that's how things worked out. Just over a 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.

About vehicles(sea)

It taking a long time to become proficient with water vehicles is pretty consistent with how the Sailor Background offers that proficiency - you spent you 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 ...)

Yes. The Training downtime activity would allow a character to gain proficiency with Vehicles (Water) after spending 250 days training at 1gp/day of cost.

However, the expanded rules for Ships in Ghosts of Saltmarsh include a number of actions that characters who lack a proficiency in Water Vehicles can participate in to assist in the running of a ship.