Most prepared commercial foods are salted. On the other hand, most raw foods (or when prepared without salt) are low in sodium, especially fruits, vegetables, nuts and grains. Fish can be low or high in sodium (check the link below), the same with meats.
Here's a list of more than 3,400 foods arranged from low to high sodium content
Foods containing most and least - Fineli
Foods high in potassium may have a taste like they are salted. When you cook foods in the vapour (placed above the boiling water), they retain most of potassium and when boil them in the water, a lot of potassium leaks out. Cookware for cooking in vapour is available...Examples of high potassium, low sodium foods:
- Potatoes, and especially potatoes with skins
- Many green leafy vegetables (spinach, cabbage, beet greens, Swiss chard, etc)
For other foods check the link.
Substances that oppose the effect of salt are diuretics — they stimulate the excretion of water. Some foods/herbs contain natural diuretics and some of them also stimulate the excretion of sodium and some of them are even available as extracts in the pill form. I will not mention any of these; it is a doctor who can say if they are appropriate for your grandpa, which type and in which amount...
Water retention in a heart patient cannot be always controlled with a low-salt diet. One thing is a limited fluid intake. Another things are prescribed diuretics. Again, it is a doctor who can say how much water, diuretics yes or no, etc. Please do not take any actions on the basis of this post alone.