I get asked from time to time by people wanting to visit Cornwall, for advice on where and when to go sea-watching down in the far South West. Now, as a comparative novice I'm not really best placed to offer advice on this but over the years I have gleaned the basics by asking the locals for advice and I've laid that out below. For a far more comprehensive description of Cornish sea-watching, see this excellent article on the CBWPS web-site here.
In general you want a strong wind for sea-watching as it blows what are normally distant pelagic sea birds closer to shore. My rule of thumb is that anything averaging over 20 mph (on the BBC weather web-site) is strong. Down on the Penwith peninsula there are two main sea-watching sites: Porthgwarra on the south coast and Pendeen half way down the north coast. In general, a westerly component to the wind is best for sea-watching on the Penwith peninsula. Anything with a southerly component warrants Porthgwarra (so southerly round to to westerly) and anything with a northerly component favours Pendeen (so westerly round to northerly). Straight west is a bit of a toss-up but probably Pendeen wins out, especially later on in the season where it gets better whereas Porthgwarra tends to be less good.
Park at the Porthgwarra car park and take the coast path west a short distance up to Hellas Point where there is a large rock offering shelter. The only marker on the sea is the Runnel Stone buoy so if something has been called which you just can't pick up then the best ploy is to wait for it on the Runnel Stone where the bird will usually be called out as it passes. In the (fee-pay) car park there are toilets and a café.
There is a free car park next to the lighthouse but no toilets. Walk around past the entrance to the lighthouse itself and through the small rusty gate and then round to the left to sit underneath the lighthouse itself where it is relatively sheltered. There are three rocks making up a reef a few hundred yards off-shore which make for excellent reference points for calling out birds.