It is 45 minutes of open Atlantic from Peniche. With a swell the crossing rolls (reviews mention waves that were “quite impressive”) and the caves are only visited in calm seas. Check this before choosing your day.
Clear sky
Indicative reference based on OpenWeatherMap data for the island’s coordinates. Whether to sail and whether to enter the caves is always the operator’s call. If you get seasick, sit at the back and watch the horizon: it is 45 minutes.