Ships are subject to so much drag that this is rarely a problem, only in emergency situations and there is not much that you can do to stop a vessel that weighs 100,000 tons or more except to run your engines in reverse and start praying to your deity. Regenerative braking for boats would be a complete waste.

There are some vessels that have single use emergency brakes, but the latest trend is to have motor 'pods' that are electrical and that can be used both for normal propulsion as well as to perform emergency stops that are quite impressive given the size of the vessels they are on. Typically an oceangoing vessel requires at least 3, but commonly 5 to 10 ship lengths to come to a full stop from moving forward under power. This is not necessarily because of limitations of the propulsion unit, but simply because stopping that much tonnage too fast would do as much damage as a collision would. With classical engines there is far more rotating mass so it would take much longer than with electrical propulsion to react before the beginning of the braking phase.

Isn’t regenerative braking reclaiming otherwise wasted energy from necessary deceleration? Running the propeller in reverse would result in having to apply equal or greater energy to regain the current speed, so it’s a net loss of energy if I’m understanding the suggestion properly.

Not changing course is good though. Regenerative braking is only good because it increases the efficiency when you absolutely must slow down, but it would always be more efficient to slow down less.

Wind, large surface area for solar

Also wave based generators that could also act as dampers/suspension and they wouldn’t steal energy from forward motion like wind would (depending on if you’re generating wind energy or using wind to buttress the batteries).

Ideally a combination of sails coupled with batteries and wave generators sounds like it would be very energy efficient.