When traveling to Helsinki, you only need one bottle of water during your trip – regardless of the length of your stay. That’s because tap water tastes so good. You need the first water bottle to get the bottle, and you can keep on refilling the bottle from the tap for the rest of your stay.
If you, for some reason, need to buy bottled water, then choose at least one of the local brands to avoid pointless logistics. The most stupid thing is to ship or to fly Evian or some other famous label from thousands of kilometres into a place of excellent tap water.
Many restaurants in Helsinki are smart enough to serve tap water as the non-gas option. If I see water bottles on the neighbouring tables in a restaurant, I might get a bit of a bitch and order specifically tap water.
And when the time of goodbye comes, don’t ditch your water bottle into trash bin. Bottles have a deposit system, and that’s the reason why about 90% of bottles are recycled. Leave your empty bottle close to a trash container, or return it into grocery store.