They are complex little animals 
They need to eat fairly constantly - a bit like a horses digestion (but on a smaller scale) they will graze 20 hours a day and pooh lots of innofensive waste (unlike your carniverous types) but like horses, GP cannot be sick.
If they don't eat for a period of time (approx 7 hours) their guts go into stasis and they suffer organ (liver usually) failure. Which is why you'll read about us shovelling Critical Care or mashed mushy pellets into an ill non-eating piggie.
Water- mine never drink it but it's always on offer. I see pigs in pet shops at the water bottle but they don't seem to have unlimited vet (I presume they have veg am/pm then dry food in the daytime)
Mine pee (loads) so they're getting their fluid from veg
Lettuce - it's iceberg you can't give them, it's too watery. But things like romaine or those little gem lettuces (mine like the red /green ones )
Oranges - they can have but they're messy with them.
Cucumber is lovely in summer
Mine love corn on the cob - but they are fussy. Corn from Tesco - meh , from Sainsburys they demonlish. Costco, nope.
Romford market with the leaves on - yay.
Main thing is not to give too much of one veg.
Brassicas (dark green veg ) can cause wing 
and there's the veg which have high levels of oxalic acid ( your watercress and spinach for example) that increases the risk of bladder stones.
Fruit can make them 'loose' and has a higher sugar level that veg (pigs are at risk of diabetes) . If you give apple, cut it upp (they can get little mouth soars) and celery (the stringy bits)
Grass is fine if it's wet (I usually sluice it under the garden tap) but not wet grass to sit on. It put them at risk having a wet belly.
They will eat their hay in the evening when they settle down to pooh eat (one of their less attractive qualities )
There's a guinea-food list somewhere - we keep doing them and updating them .