Seal Traits are as follows: (PB: Proficiency Bonus)

STR: 10+PB (+PB/2) 

DEX: 13+PB (+(PB+3)/2)

CON: 14+PB (+(PB+4)/2)

AC:  14 + Dex modifier

HP: (7 * Your Level) + Con Modifier

Speed 15 ft., swim 50 ft.

Traits

Charge. If the seal moves at least 30 feet straight toward a target and then hits it with a slam attack on the same turn, the target takes an extra 3 (1d6) bludgeoning damage.

Hold Breath. The seal can hold its breath for 30 minutes.

Arctic Home. The seal has resistance to cold damage

Actions
Slam. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) bludgeoning damage.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) piercing damage.

• You may only transform if you are in possession of your Seal coat. (A magical item)
• You regain expended uses when you finish a short or long rest. You can stay in a Seal shape for as long as you choose, but you must pass a Constitution saving throw of 12 after half the number of hours equal to your level (rounded up) or be forced to become human. You can revert to your normal form by using a bonus action on your turn. You automatically revert if you fall unconscious, drop to 0 hit points, or die. 
• Your physical statistics are replaced by the statistics of the Seal, but you retain your alignment, personality, and mental scores. You also retain all of your skill and saving throw proficiencies, in addition to gaining those of the Seal. If the Seal has the same proficiency as you and the bonus in its stat block is higher than yours, use the creature’s bonus instead of yours.
• When you transform, you assume the beast’s hit points and Hit Dice. When you revert to your normal form, you return to the number of hit points you had before you transformed. However, if you revert as a result of dropping to 0 hit points, any excess damage carries over to your normal form. For example, if you take 10 damage in animal form and have only 1 hit point left, you revert and take 9 damage. As long as the excess damage doesn’t reduce your normal form to 0 hit points, you aren’t knocked unconscious.
• You can’t cast spells, and you have difficulty carrying objects you cannot drag with your mouth. You also have difficulty communicating unless you can do so through non-verbal or written means.Transforming doesn’t break your concentration on a spell you’ve already cast or prevent you from taking actions that are part of a spell.
• You retain the benefit of any features from your class or other source and can use them if the new form is physically capable of doing so.
• Your equipment merges with your Seal form, unable to be used or accessed unless you choose to drop it before transforming.

You have a good nose. Selkies have proficiency in perception.

Your base walking speed is 30ft

Selkies mature at a similar speed to humans, but may live a lot longer, up to around 200 years.

Most Selkies lean towards Good alignments. Very few are Evil.

Selkies are Human-sized, around 5-6 feet tall.

Selkies are typically attractive and tough.  +2 Charisma and +1 Constitution.

Selkies know Common, Aquan, and can communicate with Seals.