I'm not convinced by the explanation in the first quote below, for while whales can splash, their splashes aren't huge in the context of the hugeness of oceans. A whale's splash affects the ocean far less than a financial whale can affect a stock market.
A whale is any individual or company who has enough money and power to directly influence the price of a cryptocurrency or stock, usually in a negative way. Think of a whale and their large mass. They can make huge splashes and the same concept can be applied to crypto/financial markets.
whales has become a term associated with market manipulators. Market manipulators will come into a market, buy a majority of the available positions or limit orders and drive up the price. Once the price has reached their target price, they will attempt to liquidate their position, albeit a large position which may take some time. Generally, this will cause the price to crash back down to it's inherently acceptable level.