A Cull the Weak is played by my opponent. I choose my Scuttle Crab, and they choose a Watchful Sentry to die. Does Scuttle Crab's deathknell reveal what they currently have or does it also include what they draw from Watchful Sentry's deathknell too?

When Cull the Weak resolves, your Scuttle Crab and your opponent's Watchful Sentry are destroyed simultaneously. Both units have Deathknell triggered abilities that are placed on the chain together.

According to Rule 376.3.b.1 (referenced in FAQ #9265): "If multiple players separately control Triggered Abilities that are triggered simultaneously, then starting with the Turn Player and proceeding in Turn Order, each player orders their Triggered Abilities on the Chain."

The Resolution Sequence:

1. Triggering: Both Scuttle Crab and Watchful Sentry die simultaneously. Both Deathknell abilities trigger.
2. Ordering: The Turn Player places their trigger on the chain first, followed by the non-turn player.
   • If it is your turn, you place your Scuttle Crab trigger on the chain first, then the opponent places their Watchful Sentry trigger. Your Scuttle Crab trigger resolves second (after the draw).
   • If it is the opponent's turn, they place their Watchful Sentry trigger on the chain first, then you place your Scuttle Crab trigger. Your Scuttle Crab trigger resolves first (before the draw).
3. Resolution (LIFO): Abilities on the chain resolve in Last-In, First-Out order.

Conclusion:

Whether the reveal includes the drawn card depends on whose turn it is.

If you are the non-turn player, your Scuttle Crab trigger will resolve first, meaning the reveal does not include the card drawn from the Watchful Sentry. If you are the turn player, your Scuttle Crab trigger will resolve second, meaning the reveal will include the card drawn from the Watchful Sentry.

Note: I could not fully verify this ruling against the retrieved rules, as the outcome is dependent on the turn order status of the players involved.