For speedfreaks that already own an iPhone or iWatch we would like to recommend "Waterspeed" . The app produces a pretty solid 1Hz loging and provides good feedback of speeds and runs during the session. So for record purposes it is not suited, however, but for fun rankings it will be suitable. The GPX-files can be posted directly from the smartphone to GP3S or processed by GPSResults. In the near future we will cooperate with the developer to improve the accuracy, features (like 5x10s AVG) and auto-uploading to our platform.
Please keep in mind that very cheap cellphones < 100€ will not perform very well since they use generic GPS-chipsets and antenna - the reason for outliers etc has nothing to do with the App that just uses the data from these chipsets!
Of course an Aquapac is required and pressure on the touchscreen has to be avoided in order to not start other activities on the phone! Energy saving options on the phone should be disabled.