Ian's info regarding the lack of direct service from B-o-C to LST is correct.
There is a ferry over to Wallasey Island, but the guy who operates it, Mark, is a schoolteacher so his ability to run it is restricted to running it during school hols and at weekends.
I've no idea how he operates 'out of season' - which this ride may be.
If you're thinking of using it it'd probably be sensible to give him a call on 07949 940 297. (He has always returned my calls, when I've left a message).
That'll give you access to the usual train services from Southend (after cycling a further 10 miles or so)