'Too much water' is entirely subjective, unfortunately. I keep a 'pad' (folded up piece of paper towel, really), that I not only clean brushes with but also use to dab out the excess water (and paint/wash). But sometimes, I want a little more water in the mix, and sometimes I want a lot less - it all depends. Time and practice - experiment! If you hate it, you can scrub it off and do it again.
If your wash 'puddles', you have too much water, and you need to add a little paint back in. I've had that happen, and .. yah, basically, add a little more paint in, and it'll be fine. 1/10 ratio of paint/water is a good place to start with making a wash, iirc.
Keeping a mini from looking 'dirty' when using a wash.. I use a fine brush, and a light hand, just dabbing the colour in where I want it. It will fill the cracks with almost no help at all. You might need to dry the brush and pick up some of the excess (just dab it off on your paper towel, or whatever, then dab the excess, let the brush soak it up).