Why is my cocktail watery?

Over-dilution. The most common causes: crushed ice instead of cubed (way too much surface area), too long a shake or stir, ice cubes that are partially melted before they hit the shaker, or weak base spirits.

A watery cocktail is almost always a dilution problem. Diagnostic checklist: (1) Ice. Are you using fresh, hard, dry cubed ice from a freezer at 0°F or below? Wet ice — ice that's been sitting in a cooler with water — melts immediately on contact with warm liquid. Crushed ice should never be used for a normal shake or stir; the surface area is enormous and the drink over-dilutes in seconds. (2) Shake duration. 12 to 15 seconds is the standard. Some recipes call for longer (egg-white drinks need 25 total). 30 seconds is over-dilution. (3) Stir duration. 25 to 30 seconds is standard. 45 seconds is too long. Watch for ice cubes visibly shrinking during the stir; that's the cue to strain. (4) Spirit proof. 80-proof spirits in a heavily diluted drink read as watery; 100-proof spirits hold up. Try the same recipe with a higher-proof bottle. (5) Recipe ratio. Some recipes online are simply under-spec'd on the spirit. If a Margarita feels watery, try increasing tequila from 1.5 oz to 2 oz; the ratio should be roughly 2:1:0.75 spirit-citrus-sweet.