Yes, you can ride in a boat and swim in a lake if you have a fish allergy. There will not be significant exposure on your skin to fish allergens to trigger an allergic reaction.
For fishing, gloves made of nitrile can help to prevent exposure to fish allergens on your skin. If the exposure cannot be avoided, these types of gloves can help protect you from fish allergens while handling bait and fish.
The main issue with a fish allergy is to avoid eating fish, especially the type of fish that you are allergic to such as cod, tuna, and/or salmon. Remember to always have your epinephrine with you while on the water.