You can try 40 MHz but whenever get in to trouble try 20 MHz and 15 MHz too. The solution for decrease multipath is using diversity instead of using Dual Pol Antenna.

Hi,

I'm planning a new link, that PtP 25Km total, but ~4.5 or 4.9 Km of the link are over water (sea).
I will use mimo parabolic (30 or 34 dBi gain) antennas and RB411AH device on both side at 5GHz, 40Mhz channel.
There will be a temperature difference between 10°C and 35-40°C from winter to summer.

I would optimize the link to get as more as bandiwith, I'm think of ~50Mbps.

Can you advice me about what kind of precautions should I take ?
I'm aware that will be signal fluttuations over the water. How can I mitigate it?

is NV2 good for that link?

Please let me know if I missed something on the scenario description.

thank you

It's is not good to talk about using diversity now while you don't have any calculation of the link or not try the link till now.
I recommend you to use some link planner. I attached two files about my project. one before using diversity and the other after that. You can use Motorola Link planner to open them. It also works with Google Earth.
noticed that this is all about Motorola not any other brand, but using Motorola Link Planner is useful for simulating I think.

I hope Mikrotik release software like Motorola.

You may read some useful source like CWNP books. There are some characteristic such as :

Visual LOS
RF LOS
The Fresnel Zone
Intentional Radiator
EIRP
Wave propagation

这是很重要的一个无线连接。

I would use antennas with less gain (23dB) and when installing them set it´s uptilt to the point of -3dB. For a 23dB typical antenna it´s beamwidth is 10º, so you uptilt your antenna 5º.

Another approach is to set 2 antennas on the same mast with +/- 10 meters of vertical separation and the -3dB uptilt connected to a R52Hn radio.