How old ?
We were able to get Ordinance Survey Maps from 1900's, probably still can. Phone Linghams Book Shop as they used to have them , or they may have a contact.
0151 342 7290 (Heswall)
I'm not sure how you are thinking of doing the walks, either by advertising or local friends etc. but if it is for the general public I am sure you would need insurance cover too, just in case e.g. someone fell into the dock or tripped on an old plank, they could sue you ! I think it's called 'public liability'
Public liability insurance protects you if clients or members of the public suffer personal injury or property damage because of your business. It can pay for the costs of subsequent legal expenses or compensation claims and is an integral cover for businesses that interact regularly with customers.