Put the sugar snaps in a heatproof bowl and cover with boiling water. Leave to stand for 1 min, then drain and rinse under cold running water. Shake off the excess water, dry with kitchen paper and set aside.
Put the potatoes in a saucepan and cover with cold water. Cover, bring to the boil and cook for 8-10 mins until tender. Add the peas for the last 2 mins of cooking. Drain well; return to the pan.
Add the butter to the potatoes and peas, then crush gently with a fork; re-cover to keep warm.
Rinse the sea bass and pat dry with kitchen paper. Brush a large frying pan with a little sunflower oil and heat over a medium-high heat. Add the fish, skin-side down, and cook for 3 mins. Reduce the heat to low, brush the top of the fillets with the remaining sunflower oil, turn over and cook for 2 mins until just cooked through. You may need to cook in 2 batches.
Toss the tarragon with the watercress, sugar snaps, vinegar and half the olive oil; season. Pile the crushed potato mix onto a warm serving plate, top with the fish, then the watercress salad. Drizzle with the remaining oil and serve immediately.