What @Vicko said. As a matter of fact I'm going to Vis in two weeks.
As for Montenegro I dove only northern part of coast. There are nice locations from island Mamula down to village Bigovo. Didn't venture further south.
One thing, though. In my experience, marine life is better in Vis and Croatia in general (apart from Istria) than Montenegro.
In Bosnia, there are few options, none really great IMO. There is river Una, which is crystal clear, but fairly shallow, Neretva (about the same). There is a artificial Neretva lake, but have no information about conditions. Both Neretva river and lake are cold through all year. This Saturday I should go and dive in Bileća lake, which is supposed to be clear and deep. Have no idea about plant/fish life.
Serbia.....not really familiar, dove on only two locations. One is Ada Ciganlija, which is a dammed part of Sava river, right in the middle of capital, Belgrade. The other is an artificial lake called Zaovine, beautiful scenery around the lake. But, at this moment it is completely drained, have no idea how situation will develop during this year.
Since you are beginner (at least your profile say so) I guess you're not interested in this, only five clicks from my home?
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@admikar did u dove in Krušnica vrelo?
There close by is also Izvor Klokot