Alibaug is humid for most of the year. Dress for that first and style second.
The general rule
Loose, light, natural fabrics. Cotton and linen. Synthetics are miserable in this humidity.
Layers matter more than you think. Evenings by the water are cooler than the day suggests, especially in winter.
Footwear
Sandals for most of it. If you plan to walk to Kolaba Fort at low tide, wear something you do not mind getting wet and muddy.
Evenings out
Alibaug is relaxed. Nowhere here expects formal dress, and turning up over-dressed feels stranger than under-dressed. Smart casual covers everything.
For an evening with live music or a sundowner, think comfortable rather than dressed-up.
Monsoon
Waterproof, not just water-resistant. And footwear you can walk in when the ground is wet.
The thing people forget
A light layer for the ferry. It is windier on the water than on land, in both directions.