18th May , I can join outing ! Hurray !
Yes, the 18th is a better date overall.
Last time I tried going to Marina Bay for sunrise shoot was quite complicated cuz I don't drive. Earliest train is 6am so from Toa Payoh two trains later I get out and walk fast as I can (dont run or jog anymore) to the meeting place by then no more sunrise and the sun is burning hot. So unless we're picking up each other very early in the morning you'd need to catch a bus (not recommended) or a taxi.
I'd tend to steer the group away from Marina Bay on the 18th exactly because of the Regatta. There will be at least 5000 other photographers that day with the same idea so based on past experience they camp out along the railings overnight. Then when you stroll by hoping for a good shot all the best places are already take with banks of tripods and cameras setup and waiting. Can be considered a 'chofe' or 'chuffing'. A watchman is always posted and sometimes they drag a large green rubbish tip in front so you cannot disturb them.
Alternatively may I suggest:
(1) get a friend or two to come with you to a nearby temple for Vesak day and practise street photography techniques, should be very colorful and rich with subjects
(2) gather the group for a sunset shoot (the techniques for either are not so different). Begin with a makan session around 4pm then reassemble around 6pm and setup for the shoot, stay till after the blue hour
(3) for example -- gather at Changi Village to makan, then afterwards hike on the boardwalk a bit until come to a rocky beach for sunset long exposures
(4) another example -- gather at Changi Village to makan, then catch the ferry over to Pulua Ubin, stroll inland a bit, setup and shoot long exposures at a quarry pond, rocky beach or swampy marsh
..my three cents worth, I'm fine any which way.