Almost everyone using leaded solder on a boat or shop bench will be using the much cheaper, older style, 60/40 solder with an unknown flux. The solder can be oxidized or corroded on the outside of the reel as well. I know this has happened to me and continues to happen as I do not really solder a lot any more. So only buy a limited quantity from Digi-Key, Mouser or other reliable supplier. Quite often I use Amazon but lately Digi-Key is getting most of my business. There is no minimum order with then anymore.
63/37 solder is better. It has a higher tin content. The main advantage is this is a eutectic solder. Basically this means the solder only has one melting temperature. 60/40 solder has a temperature range. Once it is hot enough to melt, it has to cool down to a lower temperature before it solidifies. An joint movement during this solidification process will result in a poor joint. Joint movement is a joint killer and this happens even with professional had solderers. You can imagine what some of mine look like. A cold solder joint will be dull colored, whereas, a good joint will be shiny.
Here is an example of what you should look for:
This is only an example so be aware that many different lengths and diameters are available. I tend to use the smaller diameter solder and if it has to be thicker for major joints I double or triple, etc. the solder and twist it to make a bigger diameter. Since this is not ideal, if you have a lot of bigger joint work to do buy the larger diameter. Kester solder has been around for a long, long time and I used to use it back in my early electronics days.The link above will take you to their site and you can download literature and specs on their products.