Tidal generators are very much a technology (one, admittedly, in it's infancy). The forces the ocean provides are generally too extreme to be casually harnessed, and even something as seemingly benign as waves redounding on the beach contain energy on the scale of nuclear weapons, and it's contained in an extremely corrosive medium (salt water). And that's not even the deep ocean movements, which are unfathomably (sorry) more powerful even then, and that's before we get to the fact that the engine driving the tides is the entire damn moon.

Something like a tidal turbine would be completely unworkable on a ship; by their nature they produce energy from the difference of forces - which is to say, they would only generate power as long as the boat is moving counter to the current being harnessed. Even in an ideal world where we're sovereign citizens to the laws of thermodynamics and have cast off the shackles of entropy, your ship would still have to output the same amount of energy as your turbine is harvesting just to stay in one spot.

There's another common idea which is to use some kind of "water sail" that harnesses the currents and drags the ships along. The first problem with this is that, unless you're about to have a very bad day, the ocean is creepily slow. Waves travel through the water, but don't really displace the water molecules themselves, so there's very little for an underwater sail to harness (side note: but do make use of underwater sails, but in the exact opposite way most people expect - sea anchors are basically enormous submerged sails, used to keep a ship relatively stationary when it's too deep to use a traditional anchor). And were we to delve deeper for some of the deep thermal currents, it can take hundreds of years (or more) for water to make a complete cycle, which would be a very slow trip indeed.

Yet another proposal I've seen is to have some sort of large sliding body, a bit like those flashlights with a rod in them that you charge by shaking up and down, only much larger. The idea is that as the pitch of the ship changes as it ascends and descends waves, some very large weight (or multiple smaller weights or etc.) slides back and forth through long sets of coils, generating tremendous if quite periodic amounts of energy which is then stored in batteries or etc. This idea has many flaws, but I'll briefly touch on three:

• The free surface effect, a mechanism through which things moving on a boat cause the spontaneous generation of so many partial differential equations that your ship decides it'll be easier just to sink itself
• The huge amount of valuable cargo space that has to be dedicated to the generating system
• The question of what to do when you're stuck on calm seas.

The best approach would be to capture the quicker currents that are close to the surface, which are are often wind driven. There's actually been huge advancements to harvest wind currents directly, usually using large sheets of cloth (or some other medium with high tensile strength) attached to long towers on the surface of the ship with these things called sails.

... Okay but joking aside, there actually have been really cool advances in using sails on cargo ships, that are extremely promising. More work needs to be done before we see them on a panamax, but there's already been some fairly eager adoption of rotor sails for smaller cargo ships! It's super cool tech that I hope can be combined with electrification to massively reduce (or if I were an optimist, eliminate entirely) the impact of shipping. Time will of course tell, but there is some cool progress being made.