Energy from the sea...

There are a couple of ways of getting energy from the sea - tidal energy, which is effectively energy from the moon, since tides are caused by the moon orbiting the earth - and wave energy, which is energy from the sun causing wind which whips up the waves.

The methods for extracting these two types of energy differs.  The big benefit of tidal energy is that it's predictable - and constant (in that the tides happen twice a day every day - well almost - although the height of the tide will vary depending on whether the sun and moon are aligned or in opposition or somewhere in between.)  I haven't seen much beyond the theory yet on extracting tidal energy - that may be due to my ignorance - but tidal barrages are likely to be big and thus nimbyism and other practical problems are likely to dominate.

Wave energy suffers from the same problems as wind energy in that if the wind drops, then the height of the waves will drop and there will be less energy available.  The converse is also true - if the waves get too big then the wave generator could generate too much power, causing electrical damage or be physically damaged.  Sea waves are hard to predict - quite a bit of work has been done making practical measurements and then creating mathematical models to allow engineering analysis to be done.  These go by names such as Pierson-Moskowitz and JONSWAP.  One of the interesting things for me is the similarity between this work and some of the more traditional signal processing that goes on in the telecomms world.

There are a few companies in Scotland working on wave energy.
[2017 update: Since I originally wrote this, the two Edinburgh companies mentioned below have both gone out of business.  You can find a history of Pelamis here at https://www.furnacecompare.com/energy-efficiency/pelamis.html - including stories of intrigue and espionage.]

Two in Edinburgh are Pelamis Wave Power (with their Pelamis device - 50 - 70 metre water depth) and Aquamarine Power (with their Oyster, nearshore 10 - 12 metre water depth.)  Another company based in Inverness is Wavegen who have built a prototype onshore wave generator on Islay.  These devices each have their own advantages and disadvantages.  They each have some interesting videos on their websites that are worth a watch.  Both Oyster and Pelamis have been deployed at the European Marine Energy Centre (EMEC) at Orkney.

It would be nice if you could simply line these devices up - Pelamis in the deep water, Oyster in the shallow water and the Wavegen device onshore, but unfortunately it doesn't work like that.  Decent size waves require a long "fetch" - and once the energy has been extracted by one of these devices that's it - you can't have the energy twice.

I happened to stop by Pelamis Wave Power this morning whilst out for a bike ride.  A couple of their big tubes were visible in the construction shed.  I wonder if these are the ones ordered by E.ON?