When you arrive to Puumala from Mikkeli direction continue driving over the bridge of Puumalansalmi towards Imatra for about 1 km, after which take a right turn to Luukkolantie. The crossing has signs Niinisaari 9, Lintusalo 19 and Luukkola 13. If you are travelling from Imatra take the same crossing 1 km before the bridge of Puumalansalmi, this time to the left.
Keep driving along Luukkolantie for about 6 km and you will see the first guide to Temola Winery (Temolan Viinitila) just before turning right to Lintusalo road. The crossing has signs Lintusalo 13 and Niinisaari 3. The ferry of Hätinvirta will take you to Niinisaari. Do brake before Hätinvirta :)
The ferry takes a break between 9.00 - 9.15, 12.00 - 12.15, 17.00 - 17.15 ja 20.00 - 20.15. Otherwise it travels continuously.