Southend-on-'Sea'?

I've always been a little sceptical about Southend's claim to be "on-sea". But when the temperature dropped this weekend to an admittedly chilly, but not exactly arctic 0 degrees, I was surprised to see the sea freeze over!

So the question now is do I write to Mr Cameron or Mrs Windsor first to point out that Southend has been taking advantage of false advertising all these years, and should be renamed Southend-on-Thames.....?