I can't recommend the Tough Charts enough for an open boater. Waterproof flip charts done for around Anglesey and LLyn Peninsula. They are the best thing i have bought for the rib.
Waves at Puffin and Caernarvon bar are very tide/wind dependant.
Bridges, there are no sand banks between the bridges, but plenty of rocks that you won't see.
You can pass through at low tide in a rib/sib but the margin for error heading north east by the white beacon and the main swelly rock is not large. Same for platters, a big flat rock that lurks just below the water at times in front of the cottage.
Below is a very useful simple leymans way of getting through and staying in the channel which is important at below mid tide heading NE. Quoted from http://www.caernarfon-hbr.demon.co.uk/swell_e.htm
Approach the South arch of the Britannia Bridge keeping the white pyramid (1½ cables East of the centre pillar of the bridge) in line with the centre of the arch. Pass under the centre of the arch (clearance is 90 feet / 27.4 metres at MHWS).
Head for a point midway between the white pyramid and Price's Point. When the pyramid is abeam you should then be on the transit given by the leading lights (two small lattice towers situated on the shore under the South side arch of the Britannia Bridge). The narrowest part of the channel in the Swellies is about 100 feet wide (30 metres) and is situated between the Cribbin Rock and the pyramid on the shore. At this point you will be passing quite close to the shore. Once on the transit, keep the leading marks in line until Price's Point Beacon is just ahead of the beam, then alter course to starboard so that you pass to the South of the Swelly Rock Perch (S. Cardinal) - distance off the Perch of about 20 to 30 metres. After passing the Swelly Rock Perch, head in for the point ahead (1½ cables East of Price's Point) keeping close in to the shore to clear the tail end of the Swelly Rock
Once past the point alter course so that you are heading just South of the Anglesey pillar of the Menai Suspension Bridge (to clear the Platters). There is a clearing line for the Platters - looking aft towards Price's Point, as long as you can see part of the archway on the North side of the Britannia Bridge open on Price's Point then you will clear the Platters. After passing the Platters (you will see a house abeam on the shore to starboard) head for the centre of the span of the Menai Suspension Bridge.
As for sandbanks at low tide. Follow the bouys tightly, especially out of caernarvon SW and from Gallows point to Puffin and you will be fine.