Yours
"
CHEAP BOAT INSURANCE YACHT INSURANCE MARINE RIB SPEEDBOAT MOTOR BOATS INSURERS BROKERS INSTANT QUOTES ONLINE SEA DOO JETSKI SALES ATV QUAD CAN AM BRP PART DEALER SEADOO SERVICE SKIS ACCESSORIES BOAT CLOTHING JETS MARIVENT LONDON KENT SUSSEX SURREY ESSEX BOMBARDIER BOATS WHITSTABLE HERNE BAY"
I would say you've diluted your placement for 'cheap boat insurance' with the addition of all the other words.
I'd suggest that Google is thinking something like 'this person wants cheap boat insurance. All these pages mention it, but this one (ie yours), comapred to some of the others covers a lot more ground and so if this person had typed in 'cheap boat insurance kent' they might be more interested in this (your) site but as they haven't it might be too specific for this searcher.
If domain name was the deciding factor then 'cheapboatinsurance.co.uk' would rank higher than 'silversurfers.net' and they don't.