I've had no problems with gold so far, even without magic item shops.
Firstly, there are the expenses. The party is required by imperial law to hand over 10% of their monetary earnings in tax. Any treasure recovered that has an identifiable former owner also requires that 50% of the haul is returned to the aforementioned-owner. Given it's a nautical campaign, this means that 50% of shipwrecks and even pirate loot is going back to the merchant company that owned the original goods. So, the earnings for a 3,000gp haul is going to be down to 1,350gp revenue, even before the party looks at bribes, hiring NPCs, replenishing non-magical equipment, etc.
Secondly, the campaign is running over a period of months or years. I've always been bothered by the fact that characters can theoretically go from 1st to 20th level in the course of less than a year. The problem is most evident in computer RPGs. I've sometimes stared at my Skyrim stats, and thought about the fact that I've gained 60 levels, ended a civil war, killed 20 dragons, become guildmaster of all thieves/wizards/fighters/assassins and built 3 customized manor houses in less than 100 game days. Given 5e's excellent downtime rules, I've changed my whole approach to campaigns. These days, the party will do a 3-4 session adventure, gain a couple of levels... and then have 40-120 days of downtime. It may take them (game) years to reach 10th level. And, given that they want a respectable standard of living (and maybe some training, and carousing, and suchlike), all that downtime is surprisingly expensive. Even a month of downtime could be 100-200gp of expense per PC.
Finally, there are the personal goals. One party member wants to captain his own ship. Another wants to build a small temple to his god. Another is trying to establish his own ship-building business. Given these aims are roughly in the area of 10,000gp each, the five person party needs to recover more than 100,000gp of loot (given the expenses above) to achieve them. That's a huge tally, and should keep them occupied for many levels.
Even the possession-less / property-less monk has a gold sink, thanks to crafting an appropriate back-story. His monastery is falling into disrepair, with no new graduates coming to learn from his beloved masters. He needs to send regular tithes back to the monastery to help repair it to its former glory. He also needs to spend gold to attract new recruits and pay for their long and arduous trip to the mountains where the monastery can be found. In return, he gains Renown (DMG option) with his monastic order. As he gains Renown, he rises through the ranks of the order. If he manages to hit 50 Renown, he'll be invited back to become the new Grand Master of the order - which is his long-term goal.