Seacastle by Tansy Rayner Roberts
This is another book that was read long ago. Therefore some of the details are unclear.
Two brothers, opposites in every way, live on the shore of Lake shimmer. When Thomas hears the call from the lake, he throws himself in without thinking. Nick, who is older and more practical, jumps in as well in an effort to save his brother. And there begins an adventure…
Seacastle is the first of a series of books written for young readers. Each book is written by a different Australian author, but all have a linking theme as the race of the Shimmaron try to reunite. Children must help them find each other and repair their ship in order to go home.
This is a very pleasant fantasy book for readers aged 8-10.