This NASA page names the various ocean ships that have helped recover NASA spacecraft. There is a general trend that later missions used fewer ships. Read the link for details, but here are some highlights:
- Mercury-Atlas 6 (John Glenn, first orbital flight): 25 ships
- Mercury-Atlas 9 (Gordon Cooper, last Mercury flight): 24 ships
- Gemini 3 (first Gemini flight): 20 ships
- Gemini 12 (last Gemini flight): 11 ships
- Apollo 7 (first manned Apollo flight): 9 ships
- Apollo 11 (first lunar landing): 8 ships
- Apollo 13 (failed mission): 9 ships
- Apollo 17 (last lunar mission): 4 ships
- Skylab 2&3: 3 ships each
- Skylab 4: 2 ships
- Apollo-Soyuz: 1 ship!
- Shuttle: We don't need no stinkin' ships.
Exactly what allowed this reduction in the need for recovery ships? I am looking for a more substantial answer than "they got better at doing it" or "they were more confidant." Which improvements in technologies, procedures, or practices allowed this to happen? Sources, please.
Edit: These are recovery ships for landing, not those which may assist launch.