Building a Blue Deck on a budget?
You should be able to build a reasonable deck of any colour for casual play using just standard commons. It won't be particularly exciting, but as long as you have a reasonably large fraction of creatures, go easy on the enchantments, and choose cards to follow the mana curve (i.e. some low cost, some high cost, most in-between), you'll be fine.

I'm not entirely sure what you mean by 'mermaids'. If you're referring to Merfolk, then yes, you should be able to build a reasonable deck based on that theme relatively cheaply. A basic idea would be 4x Lord of Atlantis, 4x Sunken City, and then your choice of Merfolk cards (of which there are lots; many are common). 

Here's a list of Merfolk to give you an idea of the possibilities:

http://gatherer.wizards.com/Pages/Search/Default.aspx?name=+%5Bmerfolk%5D