Do you have a specific desire for repairing canals, rattling collecting boxes, serving in a shop, collecting litter, planting trees, cutting undergrowth, ballasting railway lines, pushing patients around hospital, delivering dinners, taxi-ing OAPs to the shops, befriending the housebound - the list is almost endless. You haven't even offered us any skills to help narrow down the list. Your local CAB will have a list of volunteering opportunities in your local area tho.
If it has to be boaty, you could try googling some of the well known ones (STA, OYC, maybe some of the maritime museums, etc) & check out their web sites. Good luck.
STA is now the Tall Ships Youth Trust. Unlikely to take new volunteers straight into a seagoing role, unless they are a teacher or youth worker of some kind, who might sail as Youth Mentor. They do have local fundraising branches though, who might be glad of some help.