I have the exact same issue. I believe the poster above is right - they are cosmetic galls due to external irritation. Not all of my water spinach have them. I can scrape the spikes off with my finger nail. Some of my stems have them as well.
These are growing in my aquaponics system. It seems only some of my water spinach are affected. I have lettuce ...
Inspect the plants carefully, there might be caterpillars which camouflage, they would look like a part of the stem. I came across similar issue just today and could take out 6 of them after inspecting the plants.
Yoh might also come across tiny black group of dots on your leaves, not exactly sure what they are either poop or eggs, wipe them off.
Top insects are thrips: https://www.rhs.org.uk/advice/profile?pid=876 and will be causing some damage, other insect is a psocid https://en.wikipedia.org/wiki/Psocoptera and should not be a problem for the plant.