Mr Watkins said his school had been taking part in the fundraiser for around eight years.
"I'm a big supporter of the trust and we like to do everything we can to drum up support so I'm putting out the challenge to all the other schools to come down to the lake, dress in whatever you like, and take a dip for the trust."
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He said for a gold coin donation this year pupils were given the chance to vote for which costume he would wear when jumping into the lake.
"The mermaid won and I can tell you it's going to be freezing because I don't wear a wetsuit and it's quite skimpy."
Westbrook School pupils Stuart Thurlow, 8, Paige Waitoa, 8, Annalise Drabble, 9, and Paige Dudson, 9, are also preparing to make the big leap.
"My favourite part about taking part in the fundraiser is being able to support the trust and everyone seeing my costume," Paige Dudson said.
Stuart agreed, saying he could not wait until everyone saw him dressed up as a stormtrooper.
Trust chairwoman Jules McLaughlin said last year's event was so successful and they were hoping for the same level of support this year.
"We have about 45 dunkers confirmed so far and we just heard back from the local fire service who are going to take part.
We're hoping more people will come on the day because it's quite special seeing all these crazy people in outrageous costumes jumping into the lake."
- For more information go to the trust's Facebook page.