I own a sailboat, and let's just say that it's not as simple as it looks. If you want to have good sailing performance you can't just weld a mast to the deck and call it good: Half of what makes a sailboat go happens underwater, so it has to be designed as a sailboat from the get-go. Not an impossible task, but it'll need new ship types and perhaps new construction methods; it's not something you can just retrofit onto an existing fleet.

All sailboats large enough to sleep in also have an engine, and most of the time it's a 20-60HP diesel depending on the size of the boat. There's a lot of buzz currently around switching to electric auxiliary propulsion, but the benefits aren't as big as you get on cars. Main "issue" is that marine diesel engines already spend most of their operating time in their most efficient range of RPMs; you don't get stop-and-go like you get on cars. Given that, there's no regen braking to be had on a boat. Regen can still happen when you're sailing, but the available energy from that derives from the velocity squared or something, so you only get something negligible like 200W when sailing at 5 knots (which is the common case for most 30-40 ft sailboats). You see, the main limit to speed on a displacement hull (like sailboats or container ships) is hull speed, which is mostly a function of hull length. Longer boat, faster boat.

This is good news for a container ship, though, because they're much bigger and thus tend to operate at much faster speeds like 12-16 knots. I know some bigger sailboats in the 50-60 ft range routinely see 2kW+ of regen when sailing at 7-8 knots, so a containership sailing at 12+ knots could conceivably generate in the tens of kW, which could recharge batteries for when they're becalmed or in confined waters where they can't sail.

Also, a lot of newer ships already use diesel-electric drivetrains with AziPods, where the powerplant is actually a big generator, and the propellers are attached to electric motors mounted in rotating pods under the hull. Helps a lot with maneuverability and is actually more efficient. It'd be relatively easy to add batteries in there, but the main obstacle here is that it substracts available mass for cargo.

Other main point about sails on a boat is that it's very labour-intensive to manage, so I don't think current ships with their skeleton crews could do it without almost double the people.