It looks like your Tamiya paint has been thinned too heavily with water and then applied too heavily so that it has pooled around moulded details. While drying, the paint and water has separated to some extent and (possibly) dissolved minerals (as described above) or components of the paint itself have left "tide rings" or deposits.
I do concur with the others, Use Tamiya thinner, Isopropyl alcohol or as Mike has suggested, Ethyl alcohol. When using either type of alcohol, it may be advantageous to add a drop of acrylic retarder to the mix.
Water will work to an extent, but it is one of the least favourable options. If you must use water, don't use tapwater - use demineralised or distilled water.