^{This is a provisional answer posted with the assumption the hard-science tag will be changed to science-based.}

The easiest way to discourage the development and use of seagoing vessels may be to discourage associating with the sea itself: prolific littoral and neritic predator species too large, dangerous, and/or difficult to hunt even in groups could, from an early stage, dissuade your people from going anywhere near ocean coasts and instead focus development on rivers and other freshwater bodies.

The easiest way to discourage the development and use of seagoing vessels may be to discourage associating with the sea itself: prolific littoral and neritic predator species too large, dangerous, and/or difficult to hunt even in groups could, from an early stage, dissuade your people from going anywhere near ocean coasts and instead focus development on rivers and other freshwater bodies.