Some of the very best seaview (aside from HK south but for this your budget is not enough) is actually in a little known (amongst expats) area called Tin Kau-Sham Tseng-Tsing Lung Tau (if you look for club facilities, good flats, good size)
Fantastic view of the sea, Lantau Island and the Tsing Ma / Tin Kau bridge.
Else on the other side you have Sai Kung, Cleawater bay (more houses style, so no club house facilites but an "alternative" lifestyle).
Discovery bay/Park Island, not to everyone's liking, suggest you take a look first (and bare in mind you heavily rely on the ferries)
Gold Coast, not bad but does not fit your 30/40 mn commute criteria (more like 45/55 minutes if you work a regular job)
Tung Chung, some ppl like it, some hate it, true you have everything nearby but I would feel a bit constrained (same as for DB)