I can test to see if water is salty, using a TDS meter, or just a having a taste and spitting it out. The TDS meter can help you determine if a water source is fresh, brackish, or salt water.

• Fresh water - Less than 1,000 ppm
• Slightly saline water - From 1,000 ppm to 3,000 ppm
• Moderately saline water - From 3,000 ppm to 10,000 ppm
• Highly saline water - From 10,000 ppm to 35,000 ppm
• Ocean water contains about 35,000 ppm of salt.

usgs.gov

You can find out if the tide is going in or going out, much like you can get a weather forecast.
https://www.tideschart.com/Uruguay/Maldonado

I used satellite view, to spot small streams emptying into the ocean. Hopefully they won't be all dried up.

To avoid especially cold or rainy days, or just to take a recovery day, there are hotels and hostels all over the place. Supermarkets and restaurants as well. At least on Google Maps, I won't be more than 10 miles without a hotel or restaurant to get more water. There is a stretch of 20 miles without a hotel under $50. They all cater to the tourist industry, which at the moment is practically non-existent due to winter and COVID-19. Although it isn't my desire, I could do this trip without a tent, if I was assured everything was open.

Planning backpack trip in the area of Franconia Notch State Park, in the White Mountains. Here's the map

I am trying to find weather conditions, and I think this Mount Washington Observatory may be somewhat higher altitude: https://www.mountwashington.org/experience-the-weather/mount-washington-weather.aspx

Temperature change with elevation: http://www.onthesnow.com/news/a/15157/ask-a-weatherman--how-does-elevation-affect-temperature-

Not seeing a lot of snow, where we are going, based on doing a search for “New Hampshire” on this site the says it has relatively live satellite photos (Landsat Live):
https://www.mapbox.com/bites/00113/
Found site linked from http://www.citylab.com/design/2015/03/roam-the-earth-in-real-time/388252/

This site says that the weather station is NHFN1: http://www.weatherforyou.com/reports/index.php?pands=franconia+notch+state+park%2Cnew+hampshire
On the following map, you can turn on “Terrain” by hovering over “Map”: https://mesonet.agron.iastate.edu/sites/site.php?station=NHFN1&network=NH_RWIS
There I see, that the first hut is about 1000 feet above the station, or 3.3 degrees F cooler.

I later learned that the following app, if you put in the longitude and latitude coordinates, gives you the weather forecast at that very location, including altitude. Pretty awesome. https://play.google.com/store/apps/details?id=com.nstudio.weatherhere.free

Mosquitoes won't be as bad as later in the summer, but I think they are out already in late May. They don't bite below 50 degrees, or so I read at one site.
Black Flies we should worry about, according to Whiteblaze and Farmers Almanac. I'm going to see if I have a head net, just in case.
I'll be using clothes I've treated with permethrin.

Post backpack report (May 28-29-30): Nothing bit us. Didn't see any mosquitoes or flies!