Hahahahaha!

Entry level mail order MTBs and Road Bikes both start at just under $300. Get whichever you like.

I personally think I'd rather have a lowend road bike than low end MTB for "riding around blocks/pavements".

Getting a mountain bike and actually hitting trails with it is probably better cross training for swimming. It's more of a whole body workout than sitting in the saddle grinding away the miles on the tarmac.