The most common potion, restore health, can be made from ingredients native to Vvardenfell. Any combination of Marshmerrow, wickwheat, and resin will work. If properly prepared, there are no ill effects.
Another common potion is that of restore fatigue. This can be made from scrib jelly and scuttle. Using hound meat instead of jelly may make a stronger potion, but the hound meat often spoils and makes the potion effective but undrinkable. Customers often prefer potions that taste better to ones that are only slightly more effective.
A potion to cure common diseases is another frequent request. These are quite simple to make here in Vvardenfell and often cheaper as grave dust is far more plentiful (and better preserved) by these fastidious Dunmer. Mix equal parts gravedust and green lichens. Heat over a low flame until the mixture turns brown.
Notes: Daedra skin is just as effective, but very expensive. Chokeweed can be used in an emergency.
Research notes on curing blight diseases:
So far the only mixtures I've found to be even partially effective is scrib jelly and ash salts. These ingredients are not too hard to find, but the ratio needs some adjustment and the potion is not fully effective.
A simple potion to breathe water can be made from luminous russula, a shelf fungus found in the Bitter Coast, and hackle-lo leaves. Crush the mushrooms and leaves into a paste and add water slowly until the mixture is just barely liquid enough to drink.
Note: Try and find something to mix with this to make it taste better.
A simple levitate potion can be made from coda flowers and either Cliff Racer plumes or trama roots.
Note: Do NOT mix plumes with trama root.