Hi I’ve got the following phrases (I didn’t know any latin until today)
“amo aqua , egeo in aqua vivere”
I would like them to mean:
“I love water, I want to live in water”
What I got is that vivere is in present infinitive active and if I would take for example redire that is in present infinitive. is vivere correct grammatically in this case, or should it be for example “vivo”?
Also, is there a latin word/prhase for “water living mamal”?
The infinitive is correct in this case, but there are two obvious mistakes:
In the sentence “I love water”, water is the direct object of the verb. In Latin, the direct object is put into the accusative case, so change aqua to aquam.
egeo means “to lack” or “need”; it only means “want” in the sense of lacking something (which is a decaying sense of the word 
), as in “this story leaves something wanted”. A better word for “I want” would be volo from the verb volo, velle, volui “to wish/want/be willing”.
Also, while Latin word order is not very strict, you should try to adhere to the standard subject-direct object-adverbial phrase-verb format until you have more familiarity with the subtleties of word order. With these changes in mind, a slightly revised translation would be:
aquam amo, in aqua vivere volo
Also, is there a latin word/prhase for “water living mamal”?
Not that I know of. There are names for different types of animals, but I have never heard of a word to describe all water living mammals. Come to think of it, I don’t even know if there is a word for mammal. 
If I had to come up with a word for aquatic mammal I would probably say aquaticum animal pilosum, doesn’t sound very pretty to me though.
aquaticum/aquarium/aquatile animal sive marinum/maritimum animal
Realistically, they would have modified a word to make it fit, something like aquale, - is n. an aquatic animal; a fish…
With these changes in mind, a slightly revised translation would be:
aquam amo, in aquam vivere volo
I believe Benissimus meant “in aquA” (A = a with a macron signifying ablative case). Otherwise it would mean “into the water”.
Also I would choose “habitare” over “vivere” since “habitare” indicates dwelling in a place and “vivere” has the more broad sense of being alive.
Magistra
How embarrassing 
I must have been swept up by the urge to change all _aqua_s to accusative…