The UK government website for Standard Visitor Visas explicitly says
do paid or unpaid work
[some other things]
so that definitely isn't the right visa.
The more detailed guidance says (on page 22):
Appendix 3 allows visitors to undertake volunteering (not voluntary work) provided volunteering is not ...
Voluntering on board of a cargo ship can be hard as Henning wrote in a comment. On some cargo ships however once can rent a passage. I for instance found this German prospectus with a ship touching Haifa: https://frachtschiffreisen-pfeiffer.de/reisedateien/programm/Pfeiffer-Programm-Suedeuropa.pdf (2nd entry on page 4) If one can join in Haifa or only ...
Appendix V to the immigration rules says:
Visitors Appendix 3. Permitted activities for all visitors
4 A visitor may undertake incidental volunteering (i.e. the main purpose of the visit is not to volunteer), provided it lasts no more than 30 days in total and is for a charity that is registered with either the Charity Commission ...