Everyone's experience is a little different. Some notice relatively few changes, apart from things being very slow and mellow (being underwater was a good description). Others might find the 'high' to be a lot of fun and get very happy. This might all be mixed with thoughtfulness (possibly anxiety therein) and getting hungry.
In the short term, anecdotal evidence is generally that it is relatively harmless. It has a lot in common with alcohol in this regard, where usage brings out many different characters and short term effects are relatively light (as described above), provided you don't go crazy with it.
Very little scientific research has actually been done on the long term effects of repeated use. There are 'allegations', and I think that is probably the right word, of links to psychosis/paranoia/anxiety problems/possible brain damage. Nothing here is set in stone.
This is the part that is frequently argued about whenever the legality of cannabis comes up. It's very difficult to get a sensible answer in this regard - certain websites with one bias or another will tell you different things.
There are also the hidden dangers of you not knowing exactly what you are getting. It's almost equivalent to buying medicine from a complete stranger on the street. It could have been treated with all sorts of chemicals. These may interact badly with your system.
Then there's a legality issue, though police forces tend not to take cannabis very seriously in comparison to other drugs. There's a cost issue too. And once in a blue moon, someone has a really, really bad reaction to it too.