This card targets two cards - one monster in your Graveyard, and one face-up monster on your field.
"Give and Take" increases your monster's Level by the Level of the Summoned monster while the Summoned monster is on the field. If "A Legendary Ocean" is active and "Give and Take" targets a WATER monster that is Level 4 while in the Graveyard, the monster's Level is 3 while on the field, so you increase the Level of the other target by 3.
The "Special Summon 1 monster" and "Increase the Level of 1 monster you control" effects of "Give and Take" are considered to resolve simultaneously. When you activate "Give and Take", after it resolves, your opponent can respond with "Bottomless Trap Hole".