Boat tours depart from Cartagena's Muelle Turístico (the main tourist pier near the old city) most mornings, typically between 8 and 9 am. Shared lanchas are the cheapest option — expect to pay around $30–50 USD per person for a standard day trip including snorkelling and lunch. Private boat hire gives you more flexibility over timing and which sites you visit; if there are five or more in your group, a private boat often makes better financial sense.
The crossing takes about 45 minutes to an hour depending on conditions. Seas can be choppy between November and January; check conditions before departing.
For overnight stays, small guesthouses on Isla Grande can be booked directly. Some operators in Cartagena offer package deals covering transport and accommodation.