I completely agree with CommodoreH, the multimeter is essential for RC modeling.
There are models that cost as much as a breakfast (if you eat a lot even less).
However, spending more is not a waste because you will have a better product.
Attention, almost all multimeters can make current measurements for the maximum allowed by the internal fuse which, normally (for the highest scale), is 10 Amperes.
For higher currents, a current clamp meter is needed, which is much more expensive.
Although not essential, it is very convenient to have the diode test function and the electrical continuity function with buzzer.

Take heart, frustrated is the normal state for most model boat enthusiasts.

Also, it is a common thing for the boat to work perfectly at home on the table and fail as soon as it beyond reach in the pond.

If you don't own one, now might be a good time to invest in an electrical multitester. You don't need anything fancy. They are available on eBay inexpensively and have many uses beyond R/C boats.