This expression means "without any help, or without the things that you need". It comes from the reference to ships that were stranded when the water that surrounded them became too low. Therefore, the ships were too 'high'. The 'dry' refers that they were out of the water for a long time.

Esta expresión significa "sin ninguna ayuda, o sin las cosas que se necesitan". Proviene de la referencia a los barcos que quedaban varados cuando el agua que los rodeaba bajaba demasiado. Por lo tanto, los barcos quedaban 'altos'. El 'dry' (seco) se refiere a que estaban fuera del agua por un tiempo prolongado.