Water Spinach is a variety of spinach often called River spinach, Water morning glory, Water convolvulus, or by the more stranger names “Chinese spinach”, “Swamp cabbage” and “Kangkong” in Southeast Asia.
It is also called ‘Kalmi Shak’ in Bangladesh, ‘Phak Bung’ in Thailand, ‘Rau Muong’ in Vietnam, ‘Trokuon’ in Khmer, and ‘Kangkung’ in Malaysia and Indonesia
Often mistaken for Kale, Water Spinach is grown in water or on soil that is moist. It has stems that are 2-3 metres in length and are hollow and can float on the water. It has leaves that are arrow head shaped and flowers that are trumpet shaped that are white with a mauve centre.

image: wikipedia
So can guinea pigs eat water spinach, and if so, how much can they eat?
As always, lets dig a little deeper and find out by checking out its content.
Energy 79 kJ (19 kcal)
Carbohydrates 3.14 g
– Dietary fiber 2.1 g
Fat 0.2 g
Protein 2.6 g
Vitamin A equiv. 315 μg (39%)
Thiamine (vit. B1) 0.03 mg (3%)
Riboflavin (vit. B2) 0.1 mg (8%)
Niacin (vit. B3) 0.9 mg (6%)
Pantothenic acid (B5) 0.141 mg (3%)
Vitamin B6 0.096 mg (7%)
Folate (vit. B9) 57 μg (14%)
Vitamin C 55 mg (66%)
Calcium 77 mg (8%)
Iron 1.67 mg (13%)
Magnesium 71 mg (20%)
Manganese 0.16 mg (8%)
Phosphorus 39 mg (6%)
Potassium 312 mg (7%)
Sodium 113 mg (8%)
Zinc 0.18 mg (2%)
Link to USDA Database entry
Percentages are roughly approximated
As you can see water spinach is again high in vitamin c so is great for guinea pigs but it can only be eaten by them in moderation once a week.
This is because it is high in oxalates and calcium and so if they eat too much of it they can develop stones.
So in the interest of good guinea pig care, it is best fed in small amounts.
Check other types of spinach that guinea pigs can eat right here