Replacing alternator warning lamp with LED
Often the current through the alternator warning lamp is necessary for the alternator to start producing power. An LED may not pass enough current to start that process without additional resistors.

An alternator uses its own power to energize the rotor and create a magnetic field that when rotating induces voltage in the stator.

When not rotating the iron in the rotor will retain some magnetism to start the process, however the amount of remnant magnetism may not be enough to generate adequate voltage until a fairly high RPM.  In the case of car alternators I have had it require an engine speed of ~4000 RPM if the alternator light is not functioning.

When the alternator light is present the current from the battery through the light creates additional magnetism that allows the generating process to start at a much lower speed.

Boat engines typically run at much lower speeds than car engines so this is even more important. From the forum I see that the resistor values are fairly low, implying a large current when the ignition is on but the engine not running - that will be the worst case for power dissipation - when the engine is running and the light off the dissipation will be low.

With 5 ohm resistors I would expect about 1/2 amp to flow in this case, this will result in about 1.5 Watts of dissipation in each so a 25 W resistor is overkill, 5W would be adequate.  The 270 ohm resistor only needs 1/2W.

There is low risk of damaging anything but obviously you should check your wiring very carefully as mis-wiring could cause damage to the alternator, wiring or even cause a fire.