Nota! La determinazione degli insetti necessita quasi sempre di un'indicazione geografica e temporale precisa. Invitiamo quindi gli utenti ad inserire questi dati ogni volta che viene richiesta una determinazione o viene postata una foto di un insetto. I dati forniti dagli utenti ci consentiranno anche di attribuire un valore scientifico alle segnalazioni, contribuendo a migliorare e integrare le attuali conoscenze sulla distribuzione delle specie postate.
No, genitalia is not needed to identify those, but you need to have a look at the downside and the punctation as well as at the habit. And genitalia could help by identifying the species' of this genus.
The habit is shown more or less good. Looking at this photo one knows, that it is not one the bigger species (f.e. Gyrinus marinus, G. colymbus and one or two more). It can't even be G. minimus. But beside this there is a bigger number of species no one is able to identify by photo, if those photos are not taken by an intoxicated specimen and if those photos are not very big and very good.
Indipendently of this I can tell, that Gyrinus substriatus is by far the most common species, because of its very low claims. And nearly none of the others would be swimming in open water like this one. They normally hide between reeds, roots and other things lying or growing in the water. So it is very likely this species. But this guess isn't sure enough to tell to name it G. cf. substriatus. I wouldn't do so.