They could not show how masses of warm or cold water vary or how they swell or diminish, nor how their positions change with the seasons. Below is an artists rendering of a Nansen bottle. The best explanation of each type is found in the methods used to measure the two velocities. Eulerian velocity is a measure of ocean currents which flow past a fixed position - Now imagine sitting at the intersection of two streets, counting the number of cars which drive by your intersection. Now that you understand the distinction, we can look at the different types of current meters.
Lets talk about surface drifters.
The following image is a picture of a Holey Sock, a subsurface drifter. To get a vertical profile of the current, several instrument stations are attatched to the mooring line. Just like the Lagrangian current meters, the eulerian current meters have a radio transmitter which sends the data to satellite receivers. This is a photograph of an S4 current meter used for shipboard observations. This is a simplified diagram of a subsurface moored current meter.