It's obvious for any reasonable person that it is going to be litterally impossible to get into Evian itself, neither from water nor from land.
Though, if you still consider the only challenge that is worth trying is to disturb the pourparlers, here are some envisageable strategies to try to reach the "Palais des congrès":
The thing you have to keep in mind is that security forces will be massive enough to stop any attack launched by a compact group of demonstrators.
That's why, the best chance to succeed would be to launch a simultaneous attack of several tens (preferably hundreds) of individuals coming from all directions by any imaginable medium.
Just imagine 100 swimmers + 100 runners on the east road + 100 on the west one + some additional hundreds rushing down from the forest dominating the town. Not any securtiy deployment would be able to contain such an assault. They would need about 3-4 cops to stop just 1 runner, 1 boat to catch 1 swimmer, etc.
Let's be simultaneous and "atomised"; a sudden and decentralised attack is the best chance to succeed.
Now, let's study some tactical details according to the direction your plan to attack from. Both these strategies are excellent and could give surprisingly good results:
If you choose to attack from the land : the town is dominated by a forest stretching on a gentle slope. It's dense and dark enough to make detection of numbrous individuals difficult. A big amount of activists running in these trees would be the most efficient strategy of all envisageable solutions.
Concerning the coastal road. The part coming from East is litterally "on the lake", there's not more than 5 meters sidewalk between the road and the lake.
The one coming from West (Geneva) is a bit more interesting as there's a range of public parks, private properties with gardens and public swimming-pool between the road and the lake. You could count on this stripe of grass, houses, beach, etc to reach Evian more easily, but I guess it will probably be blocked some kilometers away.
If you prefer attacking from the lake : as I said earlier, you can choose between swimming, which could be very efficient if it's made by a large enough amount of spread swimmers (imagine how difficult it is to catch someone from a boat, mainly when the swimmer does everything he can to escape from the boat) or try to reach the shore (made of blocks of rocks, don't forget your shoes) on any imaginable embarkation : surf board, raft, pedal craft, windsurf, submarine, kayak, inflatable dinghy, etc.
As you can see on the following picture, the "Palais" is not more than 100 m from the shore.
If you don't feel like carrying your own boat from home, you can "borrow" one of the numbrous boats of all types parked in the small harbours spotted everywhere on the coast.
Types are:
- boats moored at a quay. Generally you can get them simply by detaching the ropes at the front and back. I wouldn't recommend this solution, because you won't have any medium to sail them and they will be too slow to escape the cops.
- on the other hand there are hundreds of smaller boats (sail or motor) parked on the ground, some meters from the water. These ones can be stolen as easily as the bigger ones, but can be much faster with a few enthousiastic rowers.
- last choice : the numbrous windsurfs stowed in kinds of shelves can become yours if you have powerful enough pincers to cut the chains.
Don't hesitate to lead a "harassment" attack! It's the most efficient weapon for the unarmed ones.