Trieste: boat trip to Muggia, starting from Molo Bersaglieri:
http://www.triestetrasporti.it/index.php?trieste-muggia-giornaliera-annualeIf you have a car, you can go to Muggia along the coast, there are enough spots to take photos of vessels docked at Trieste oil terminal and container terminal, too. When in Muggia, go on top of the hill above, the church at Muggia Vecchia, to have somewhat of a bird view of the port, and also of tankers at the anchorage, but you will need a good zoom lens for that.
Don't forget that Trieste is close to Monfalcone (30 minutes drive), and there's a shipyard there that's just launched yet another big cruise ship; the point to take photos is just opposite the yard in the south, near the marina, but it's the off limits zone (old defunct camping site, chained and fenced), so not easy to reach.
Koper, simple: go to the port, following the signs. The containers ships will be almost close to touch.
I don't know anything about Reijkja, but if it's Rijeka you have in mind, then just walk along the southern pier/wave breaker downtown, or go east, to the last station of the bus No. 6; close enough is an old military bunker (you might think, as the rest of the population does, that it's German - it's not, it's Italian). There's a nice terrace on top of it, with a good view of Viktor Lenac repair shipyard and everything in it. I would not advise to go much more closer to the yard, as the last two tourists that were taking photos of that American command ship there were arrested. But, having been arrested by the Croatian police myself, I can tell that it's not bad. (I didn't need my front teeth, anyway...)
Rgds
Vlad