Answer:
Large boats are unable to float in the small waterways of South Africa.
Step-by-step explanation:
The phrase shows that South Africa has very limited snowy roads, as it has several small streams and insufficient amounts of water to be able to float large vessels that need very large quantities of water.
The phrase shows that there are tidal streams, which are larger streams close to the ocean that can accommodate large vessels, but this statement is made only as a reference and not as a way of expressing the main idea of the phrase. With that, we can say that the main idea of the frse is "Big boats cannot float on small waterways in South Africa."