This page links to some measurements of sea, estuary and river temperatures.
The system uses a semiconductor sensor - DS18B20 - which can measure temperature to ±0.5°C accuracy from -10°C to +85°C (3 sigma error across devices). It has a resolution of 0.0625°C (1/16°C).
The sensor's response time was measured by moving it between two water-filled temperature baths at 1-minute intervals. The baths were stirred continuously to simulate the sensor being towed beside a kayak. The graph below shows three such cycles. The temperature moved to within 50% of the actual temperature in about 4 seconds, and within 90% in about 10 seconds.
Temperature (°C) - Time (mm:ss) Graph. Click on image for larger version. Measurements (shown by dots) are made every 2 seconds.
The sensors are interfaced to a microcomputer (a Raspberry Pi Zero or an Arduino) which interrogates the sensor every 10 seconds. The measurements are stored on an SD card. The microcomputers have a small LED display which shows the date and time together with the most recent temperature measurement. The Pi Zero version also runs a web server so that recent measurements can be examined and downloaded.
A small weight was attached to the sensor which was dangled into the water. The electronics were stored in an orange waterproof bag on the deck of the kayak.