An Indian Ocean fleet would be harder to surprise

Could Japan have attacked a substantial Royal Navy fleet in the Indian Ocean at the same time as Pearl Harbor?

Not with comparable surprise or strength: carrier numbers, watched approaches, submarine range, and resupply favored a later attack on an alerted fleet.

A British fleet in the Indian Ocean would present a different operational problem from Pearl Harbor. Japan owned only so many carriers, and diverting part of the striking force would weaken the Hawaiian attack. Approaches through the Dutch and British observation network were harder to cross unseen, while submarines operating from French Indochina faced long distances and their own allocation tradeoffs. The more realistic sequence was to strike Pearl Harbor, return and resupply, then move west. By that time the Royal Navy would be alerted, likely at sea, and preparing its own operations rather than waiting in an unsuspecting harbor.