> It’s like eating a ball of water.
We are talking about different fruits here: Prunus domestica, The Common or European plum, and Prunus salicina, the Japanese plum are, by far, the most cultured.
European plums are sweet, acidic, fleshy and firm, with a rich taste.
Japanese plums are sweet, acidic, have a less complex taste and are soft and --watery-- by nature.
The best Japanese plums are decent fruits with an useful trait. Matures earlier, so if you buy plums in June are most probably Japanese plums. You will have a sweet-acidic ball of water all the times.
Not much unlike cherries that mature at the same time, everybody loves and are basically sugar and water. The good part is that you can have plums off-season. They aren't the best, but is the best plum that you can have in May-June; and is the only that you can culture in warm areas. Their season extends to July and maybe August.
European plums --require-- cold. Without a minimum amount of frost hours in winter will not bloom. A few mature as early as June or as late as October, but the common season ranges from mid July to mid Sept. Those fruits had much more time and sun to develop their flavor and are more diverse and different in nature, with several subgroups. Some are mostly acidic and some store a lot of sugar. Some have a very nice trait, a 'clinging seed' (can be detached cleanly from the edible part). Unlike Japanese, they can be dehydrated and stored as prunes for many months. In sum, a much more versatile fruit.
And there are the new pluots an plumcots also. Hybrids from plum and apricot that taste like very good european plums.
So you need to educate yourself in what to expect, or ask in your grocery. There are lots of different fruits in the Prunus genus.