It's difficult to determine that chances of fishing leviathan's amulet due to the lack of data.
Here's the amount of water elementals it took for some achievement hunters to get leviathan's amulet. I collected this information from what was shared on TibiaLD's Discord server.
10000
700
400
160
38800
13446
Which makes an average of 11,634 fishing attempts. Even though this is just a very small amount of data, we could infer the chances might be around 0.01% (or 1 in 10,000 water elementals). Cases such as Rado's seems to be widely known because they wanted to share how unlucky they were. However, when asking people around how many water elementals they had to fish to get the amulet, we find some very small numbers.