Swiming wise I don't test the water I just jump in. But drinking wise I test my water a lot because it comes from a well that isn't very deep. The test kit I use cheaks for heavy metals, bacteria and toxins from pesticides. Around here I don't have to worry about heavy metals and pesticides too much but I really worry about bacteria and parasites. I knew a guy that died from Beaver Fever (parasites) it was a long and painfull death for him. Cheers and happy first weekend of 2021!
Depends what I'm testing for. For temperature: to swim - dip my toes; at home - fingers under the tap. To know if dam water is drinkable, fill a new bottle with a litre and deliver it to a lab to test for pollutants and bacteria.