Average temperature in Croatia (Split) in May: 20°C, 67°F. Sea temperature in Croatia (Split)in May: 17°C, 63°F. Average rainfall in Croatia (Split) in May: 62mm, 2.4in. Note: May is a good month for an early season holiday in Croatia, before the summer crowds arrive.Resorts open for business and island ferries start running according to summer schedules.The weather is warm along the coast, though there are still rain showers on around 10 days of the month.ote: May is a good month for an early season holiday in Croatia, before the summer crowds arrive. Resorts open for business and island ferries start running according to summer schedules. The weather is warm along the coast, though there are still rain showers on around 10 days of the month.