During this extensive migration, Erectus probably used crude rafts to cross rivers; but, there is no proof... he could have straddled logs - or crossed further upstream. To prove that Erectus built "boats", one needed to find evidence that he settled on islands that could not be reached any other way.
At first glance, the dots on Java would appear to constitute that proof; but Erectus lived during the "Ice Age" when sea levels were 100 m or more lower than today and the big islands of Indonesia - including Java - were joined to the mainland, forming the Sunda peninsula. As a result, Erectus could walk all the way to Java.
However, the islands to the east, like Bali and Lombok, shown below, have always been surrounded by deep sea channels which prevent the migration of land animals ( and people ). In particular, the channel between Bali and Lombok forms a natural boundary - called the Wallace line - seperating the animals and plants of Asia from those of Australia/New Guinea. Settlement beyond the Wallace Line requires the use of boats.
The Island Corridor beyond Java
In the 1990s, stone tools dated to 800 Kya and typical of Erectus were found on the island of Flores, several islands beyond Bali. Reaching Flores entails crossing several channels totalling over 60 km. This was proof of seafaring ability by out ancient ancestors.
Large rafts like the one shown below were probably used. Later, in 1996, the First Mariners Project replicated these crossings.
Torres Strait islanders on a bamboo raft, 1906
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