In reply to ripper:
> (In reply to Mike Redmayne)
> [...]
>
> you think so?
>
> so it's also inconsistent (and therefore wrong) to say:
> the truck has twin axles, but it's a twin-axled truck?
> the rifle's barrel had a small bore, but it was a small-bore rifle?
> etc?
Yes, I think so, though I suspect we really need JCM to come tell us the rules!
Sea cliff is a compound noun, and Oxford says:
"In the past, these sorts of compounds were usually hyphenated, but the situation is different today. The tendency is now to write them as either one word or two separate words. However, the most important thing to note is that you should choose one style and stick to it within a piece of writing. Don’t refer to a playgroup in one paragraph and a play-group in another."
Or are you having 'sea' as an adjective?
Your other egs are different. Twin-axled I think is right, like small-minded. Small-bore is also right according to the OED (though I think you also find smallbore), but in that example you are moving from adjective plus noun (small bore) to adjective-noun compound.
Though looking at the online OED, I'm afraid they go for sea-cliff. Yikes. But the most recent source they quote for this is 1876, so I'm having sea cliff as the modern way (I'm sure Nicholson Baker, in his excellent essay on the history of punctuation, has it that some compounds lose their adjective as time goes by and people get used to the compound). Plus see the quote above. If you really want to do your head in, my concise OED has sea breeze and sea-breeze, with different meanings.
Anyway, looks as though the OP can do what he wants (though seacliff certainly looks wrong). Just be consistent.