Why does my very well marine turn my tub blue? ...

Okay so long story short..?
Answers:    It is most likely copper, it could be smoothly occurring in the sea or if your pipes are copper it could be from them. You really need a question paper done on the water right from the okay before it enter your pipes.

There are many different types of filter that will remove copper and other metals commonly found in resourcefully water.
Advice just about a plumbing situation PLEASE!?
Are you sure it's blue and not green. Green stain resources acidic river is eating away at you copper pipes. You want water treatment to correct problem.