If it were funded by taxation then the government would want control and they would start closing lifeboat stations and cutting income (look at what they have done to the coastguard helicopters). Far better to have it run by people who know what they are doing and funded by people who care.
A pal of mine was on the lifeboats in Devon. They used to refer to people who got into trouble in ill-equipped boats as "the birmingham navy". It's quite a regular occurrence for them to have to rescue people several miles offshore on a jetski or lilo.