A sit on, you are going to get "nowhere fast". If you are looking to cover any distance, you will need a sit in. People do cross oceans in them ya know. If you have paddling skill with your sit in and with the inherent low center of gravity a sit in provides, whitecaps and 20 knot winds are not hard to handle, a good exercise in stamina.
Most of the kayaks I see out east in the open waters are sit in's. The only sit on kayaks I have seen out east were in Carmin's River before they closed Glacier Bay Sports out there and posted the property off limits to everyone.
With a sit on, they are super wide, hard to flip, and you can bring a ton of gear to fish all day long. The plastic boats also weigh a ton. If you can launch close to your fishing grounds, probably the way to go though if fishing is your primary purpose for being out there. If you do need to travel on the water, upgrade to a foot pedal powered sit on. Pricey but if you know the cost of power boating, kayaking is pennies.
You can fish with a sit in but you do have to know what you are buying beforehand, something beamy with beginner type stability would be best. The intermediate and advanced sit in's need the paddle in hand for balance in busy waterway's. Not impossible though, for the experienced.
With either type of boat.....you need to be proficient at self rescue. It is hot outside and the water is warm...this is the time to practice self rescue and all the other handling skills you will need.
I agree with you. Like you said,"With a sit on, they are super wide, hard to flip, and you can bring a ton of gear to fish all day long. The plastic boats also weigh a ton".
Normally it takes me about 30mins to paddle 1 mile out on the ocean with the winds on my favor. If winds are against me, it takes me even longer than 30mins. I love my sit top yak, as I've been thru some ruff waters and thank God my kayak has been there for me when I really felt at danger of capsizing.