Hey, I would not recommend march as if you dont get good weather it could affect your whole travel, disrupted ferries, or bridge connections, end of March is better but I would recommend April tbh.

CROATIA in April: Guide to visiting Croatia in Spring 2025

Even April can be a hit or miss, highest temps going to 25c but in sun can be beautiful but you will not get the summer vibe, September would be best equivalent to April , less people, but the sea is warm and with 27-30 great for sunbathing and swimming. April is better for exploring with maybe some sunbathing but no swimming unless you're from the Nordic countries.

Croatia in September: What to expect in September, what to wear, weather, 5 Day September Itinerary

How is the weather in mid March?

Rainy, Windy, can get some sunshine, 8-12c on the coastline, not really warm, if you go southern Croatia, Dubrovnik, Korcula, Hvar, you can get some nice weather.

Dubrovnik 3-Day Itinerary : What to see and do

Is it a good time to visit beaches or island hop?

If you just want to explore and not enjoy the summer things like sunbathing or swimming. From May the sea starts to warm up slowly. Its a totally different vibe going during the summer months and being able to enjoy the sea compared to just exploring. If you got a choice of only one I would go in May or start of June to get less people but still be able to go for a swim.

I'll probably club Albania with the trip too, do they have a similar weather?

Similar weather warmer by 2-3 degrees c

I would suggest checking out this database of most asked questions by other travelers coming to Croatia: Croatia Travel Tips FAQ