This deed is derived from Nigel’s first yacht share in 1999 and was prepared by a maritime lawyer (the son of one of the partners). Since then the RYA have published their own draft deed which RYA members can view here:
https://www.rya.org.uk/knowledge/buying-owning/syndicates

Some elements of the RYA deed have been incorporated into this document, and it is open for discussion and modification prior to formalisation.

Another source for a ready made deed which can be reviewed by a lawyer for a small fee is available from:
https://www.netlawman.co.uk/d/boat-sharing-agreement

Another document only source is:
https://www.sharingcontracts.com/product/d2-boat-share-buy-shared-use-contract-small-boats/