Coding wordpress pages – Need help

Ok well basically I’m trying to code a page in wordpress. The page I’m working on is live at http://www.ourfortwayne.org/fort-wayne-parks/fox-island/

Basically – I’ll code in what I want with <div> tags and etc (experienced in CSS). Well after I save and view it – the page looks great. Well if I go to edit the page – all the code changes, for example <div> tags change to <p> tags. And due to this, once i save it, the page is all messed up. So I have to fix the code EVERY time i want to edit a simple paragraph.

Anyone know the problem?