Shipboard attire for most expedition cruises is casual. Comfort is a priority, so wearing casual clothing, layered for warmth in some destinations, is ideal. Many companies stress packing light so "dressing up" is of little importance.
Sportswear is acceptable, perhaps even in the evenings for dinner on some vessels. Plan to wear rubber-soled, flat or low-heeled shoes for activities on deck, and it's suggested that you bring a windbreaker for places with more moderate temperatures.
For lines that do enforce a dress code, evenings are generally informal and elegant casual attire is acceptable. Coats and ties are not typically required.
Your Vacations To Go travel counselor can confirm the guidelines for your specific itinerary.