You can't grow lettuce without light and warmth, so we grow our lettuce on the south coast of England because it's sunnier and warmer than anywhere else in the UK.
In fact, the south coast has 10% more light than inland areas, and for every mile you move away from the coast the light decreases by 1%. It's smart to grow lettuce by the sea because the sea reflects the light and acts as an insulator, keeping the surrounding land warm.