Here ‘pressed’ means ‘compelled to serve in the armed forces.’ The modern term is ‘conscripted’ or ‘called up’. A volunteer joins of his own free will and so is much more likely than a pressed man to be an efficient sailor or soldier.
The same applies in a wider sense. Mr James, Mr Turner, and Mr Roberts were all members of the committee of Tennis Club. Mr Turner had offered his services; the other two had been ‘talked into it’, persuaded against their wishes. What happened? Mr Turner did more useful work for the Tennis Club than the other two put together.