According to Rebbi Yishmael the prohibition of Tosefes Shevi'is learned from a Halachah l’Moshe mi'Sinai and according to Rebbi Akiva it is learned out from a Pasuk.
Rebbi Ashi says that Rebbi Gamliel learns Tosefes Shevi'is from the Halachah l’Moshe mi'Sinai and it applies only while the Beis ha'Mikdash is intact. (1)
It is forbidden to water a field on Chol ha'Mo'ed from a Kilon (a deep well) because it takes strenuous effort. (2)
Pools of water in a field that filled up before Yom Tov are prohibited to be used for the field unless there is a stream of water running between the pools. (3)
It is permitted to water vegetables on Chol ha'Mo'ed for the purpose of eating them on the Mo'ed but not for the purpose of enhancing them.
It is forbidden to dig new holes at the roots of grape vines on Chol ha'Mo'ed but if there are pre-existing holes that got filled up with earth it is permitted to dig them again.
Rebbi Elazar ben Azaryah says it is forbidden to dig a stream on Chol ha'Mo'ed and on Shevi'is. (4)
It is permitted to repair a stream on Chol ha'Mo'ed by deepening it from one handbreadth to six.
A BIT MORE
1. The prohibition of Tosefes Shevi'is is Rabbinic in nature now that the Beis ha'Mikdash is not intact and Rebbi Gamliel and his Beis Din rescinded the Rabbinic prohibition.
2. According to Rebbi Yochanan the prohibition to water a field with rain water on Chol ha'Mo'ed is a Rabbinical decree lest one end up watering the field from a Kilon (deep well). According to Rebbi Ashi it is prohibited because the rainwater sometimes becomes Kilon water when the well is no longer full.
3. There is a dispute between the Amora’im in the case of a stream running through the field which is not large enough to water the majority of the field. They argue whether it is permitted to water the field from pools of water in the field.
4. On Chol ha'Mo'ed it is forbidden to dig a stream because it is strenuous work. There is a dispute between Rebbi Zeira and Rebbi Aba Bar Mamal regarding why it is forbidden to dig a stream on Shevi'is. One says because it looks like hoeing and the other says because it prepares its banks for planting.
Rebbi Yochanan says that Rebbi Gamliel learns out Shevi'is from Shabbos. Just as there is no concept of Tosefes Shabbos so too there is no concept of Tosefes Shevi'is. Tosfos asks that there is indeed a concept of Tosefes Shabbos! It is a Mitzvah to extend Shabbos both prior to Shabbos and after Shabbos is over. Tosfos answers that the Mitzvah of Tosefes Shabbos is for a minimal amount of time and Rebbi Gamliel would acknowledge that a minimal Tosefes would apply to Shevi'is as well. However the Tosefes starting from Pesach and Shavuos was rescinded.
It is permitted to draw water in order to irrigate vegetables so that they will grow and become edible on the Mo'ed. But if you don’t plan on eating them on the Mo'ed and you are only irrigating them for the purpose of enhancing them it is forbidden (Shulchan Aruch OC 537:4).
For the purpose of preparing food for the Mo'ed even a very strenuous activity is permitted. However if you are not planning on eating the vegetables on Mo'ed it would be forbidden to water them even if drawing the water does not require you to exert yourself (Mishnah Berurah).