The wattage rating just means the speakers are able to handle up to 25W RMS without burning out the voice coils.
You can replace with any speaker rated 25Ws or more, sort of doubt your stereo can output more than that. Just because a speaker may have a higher wattage rating, does not mean it requires a higher wattage amp.
You will also find most replacement speakers will be rated 4 Ohms for 12V mobile applications, that is normal and should be fine.
Being it is an outdoor speaker, limit your search to outdoor or Marine rated speakers like you would find used on boats. Marine speakers are designed to be able to cope with life outdoors with UV and water.
Non Marine speakers are often built with paper cones and low grade foam surrounds that can't handle constant UV/water exposure.
