Mobile internet for non-Croatian citizens

[m-a-r-t-a-s]

I'm going to Croatia for holiday (yachting) and I want to have an internet access (especially due to weather forecast). Due to yachting I need a mobile internet (GPRS, edge). Roaming doesn't come on force (too expensive) so I want to buy a prepaid Croatian sim card. Supposed volume of data is at most 20 MB per staying (one week, Saturday-Saturday). Data transfer speed is not important (GPRS is sufficient).

Decisive and important is:
- cost of "empty" sim card
- minimal amount of credit (what will I do with unexpended credit?)
- validity of credit (possibility to use unexpended credit next year)
- good signal coverage (Kornat area)
- easy possibility to buy sim card

Can you help me please choose optimal tariff? Thank you