In reply to PaulJepson:
I climb at swanage, firstly where are you intending to climb? quarries Hedbury Winspit and Dancing Ledge are the usual go to if conditions aren't right. Guillemot Ledge and Subluminal and Lighthouse Cliff also have a fair amount of non tidal climbing across the grades.
Everywhere else however needs more care.
first I'd look at the tides and decide if its a neep or spring. During neeps there is virtually no significant tide at swanage due to the double high so you can likely ignore tide. At springs the tidal range is important so when is low tide?
Swell at swanage is very dependant on wind direction. If there is no swell then then areas of Boulder Ruckle around marmolata buttress and Cattle Troughs can be climbed at any state of tide.
Swell is best when there is a northerly or a southerly wind just because the cliffs. Winds from the South West are the worst and at wind strengths > 15 knots I'd worry about constructive interferance of the waves. I've seen the waves top the cliffs at Cattle Troughs and you definately wouldn't want to climb at Boulder Ruckle in a brisk south westerly.
Fortunately we don't get any swell from the Atlantic because the fetch isn't long enough.
In all cases make sure you look at the sea state before you abseil into any of the cliffs and always leave a static line in place so that if you've got it wrong you can get out quickly. And its a long rope climb so make sure you have protected the static rope from abrasion, it could easily wear through on even a rounded edge - I have seen that happen!