I stayed here in August this year. The property grounds are beautiful and treated this Titanium guest really well (breakfast was included in addition to points plus an upgrade to a villa). Having that said, I do recall reading reviews that rooms weren't ready on time and that the boat transfer from mainland was using one of those local boats instead of the Marriott boat (apparently it was sent back to the manufacturer for some repairs). I just used a regular transfer from Kuala Besut after reading about the boat swap.

I wouldn't recommend staying in January. Whilst it may not rain but the winds can be very strong with huge waves. But the Marriott is facing west (it's the northeast monsoon, so Long Beach is closed during the monsoon) so it shouldn't be hit that hard (transfers from mainland however would require you to traverse the open seas).

You can hire private water taxis from the Marriott jetty. If you are a bit adventurous, you can also hike over a small hill to Long Beach (wouldn't recommend swimming there due to boat traffic and less than great water quality but the water is very blue and clear) for some non-Marriott life e.g. bars, restaurants, fire show etc.. The hotel is also about 5-7 minutes walk from Coral Bay (wouldn't recommend swimming there either due to boat traffic and water isn't the best there either). Do note that the path to Long Beach is not lit up at night and all water taxi charges are doubled after 7PM (if you have a big group, you can negotiate for a lower price). I have done the night hike across the hill twice (it's unlit but there is a clear path, you aren't hiking in the jungle).

If you want clear blue water with minimal boat traffic, you can take the stairs from Marriott property down to Romantic Beach (stairs is at the far end of the resort instead of the one by the pool). That beach is truly a perfect Perhentian beach (only accessible to public via boat or Marriott guests).

There's an in house diving center at Marriott but it costs 3-4 times compared to the rest of the dive centers around Perhentians. But the Marriott dive center is more a "valet" dive center where everything will be ready on the really nice boat for you and you just jump into the water with your gear. In other dive centers, you'd need to carry your own tank/fins/mask etc. and do a short walk from the dive center/shore to the boat. I forgot what Marriott charges but you can easily do a fun dive for about RM90-100 ($22-23USD) plus another RM10-20 ($3-4USD) for equipment rental just outside of Marriott if you don't have one yourself.

F&B prices are quite reasonable considering the location. I remember the buffet dinner was about RM100 or so ($23USD) and it had a good spread. There's another restaurant by the pool that serves seafood/pizza and I don't recall that being that popular with guests but the dinner buffet was popular with the guests and even guests from other hotels.