The ships that ply the route are seaworthy, but just about serviceable because they are inexpensive, state-owned means of transporting goods and people to and from the faraway islands to mainland India. But that’s not the biggest problem for your plans. July isn’t merely off-season in the Andamans. Since nearly everything in the islands, from ferry services to water-based activities depend on how safe it is to be at sea, conditions will be as unpredictable as the intensity of the rains. Services can get disrupted, far-flung islands become inaccessible, most water sport operators shut shop, and other than some hotels in Port Blair, nothing can be promised with any certainty. As such, my only recommendation is that you reschedule your trip.