When a ranged weapon attack hits you, you can use your reaction to reduce the damage by 1d10 + your Dexterity Modifier, provided that you had a free hand at the end of your last turn. If you reduce the damage to 0, you can catch the missile if it is small enough for you to hold in that hand.
Your swimming speed is equal to your walking speed. Additionally, being in water or in zero-gravity environments doesn't give you disadvantage on your attacks.